DE69332126T2 - Belt drive for a printer controlled on request - Google Patents

Description

BACKGROUND OF THE INVENTION

The invention relates to direct thermal and thermal transfer on-demand printers, and in particular to direct thermal and thermal transfer printers for printing tickets, labels and pressure-sensitive stickers. Some aspects of the invention also relate to printers using other printing techniques such as laser printing, LED printing, etc.

Direct thermal and thermal transfer printers are well known in the art. For thermal transfer printing on non-heat-sensitized materials such as paper or plastics, a ribbon coated on one side with a heat-transferable ink layer is placed between the media to be printed and a thermal print head with a row of very small heating elements. When an electrical pulse is applied to a selected subset of the heating elements, local melting and transfer of the ink to the paper beneath the selected elements occurs, resulting in a corresponding row of dots being transferred to the media surface.

For direct thermal printing on heat-sensitized materials, no ink ribbon is used and the heating elements act directly to produce a chemical or physical change in an ink layer on the surface of the material. The remainder of this disclosure deals with thermal transfer printing, but it should be understood that many aspects of the invention are also applicable to direct thermal printing, Laser printing, LED printing and perhaps other printing techniques.

After each row of dots has been printed, the material or print head is repositioned to position the print head over a subsequent location, the ribbon is repositioned to provide a fresh layer of ink, and the selection and heating processes are repeated to print a subsequent row of dots. Depending on the number and pattern of heaters and the direction of movement of the head and paper, regular arrays of dots can produce individual characters or, as in the preferred embodiment, successive rows of dots are combined to form complete printed lines of text, bar codes or graphics.

Applications for such printers include printing individual stickers, typically pressure sensitive stickers, tickets and labels. Pressure sensitive stickers are usually supplied on a continuous web of release material (e.g. wax paper backing) with a gap between successive stickers. Tickets and labels can also be supplied on a continuous web with individual tickets or labels identified by a printing indicia or by holes or notches punched therein. Tickets and labels can also be supplied on a continuous web in the same way with the individual tickets or labels defined by a printing indicia or by holes, slots or gaps punched therein.

An optical sensor may be used to align the printed image with the leading edge of each label. The optical sensor includes an illumination source such as a light emitting diode ("LED") or incandescent lamp and a photodetector such as a photoresistor, phototransistor or photodiode. The illumination source and photodetector typically operate, but without limitation, at an infrared wavelength. In the preferred embodiment(s), the sensor is set through the web so that it responds to the change in the relative opacity of the backing and label materials or to a hole or notch punched therein. In other embodiments, the sensor reflects light from the back of the web and responds to an indicia printed thereon.

Such printers may also be designed to allow individual labels to be removed as they are printed. The print head may be designed to advance the web and ribbon by the length of the label separation gap plus a substantial fraction of 1 in after each label has been printed and before stopping to remove the label, in which case the web and ribbon must be retracted an equal distance before the next label is printed to avoid leaving an unprinted area on the label.

The current flow to each heating element during heating is relatively constant and is determined by the mains voltage and the electrical resistance of the heater. The energy per printed dot for uniform ink transfer is a function of web speed and average printhead temperature. When printing individual labels, the web speed cannot be constant, but can be accelerated and decelerated smoothly to account for the inertia of the mechanism. This requires changes in heating to maintain uniform print quality across the areas being printed during speed changes.

Such printers should complete individual labels as quickly as possible after receiving data for them. Printing a label requires three steps: receipt by the controller of a label description in a concise label-describing language that describes the known objects to be printed, such as text and bar codes, but not the dot patterns that make them up; formation by the controller of the label image in a bit map memory where the bits on the card correspond to the physical dots in the image; and transfer of the dots that make up the label image from the bit map to the print head, activation of the print head, and feeding of web and ribbon as previously described.

The thermal ribbon can be fed from a supply roll prior to printing and then wound onto a take-up spindle after use. Some prior art thermal printers use a slip clutch to maintain tension on the ribbon take-up spindle. The slip clutch produces a constant torque output on the ribbon take-up spindle. The slip clutch thus does not compensate for the decrease in tension due to the increasing radius of the take-up spindle. There are other disadvantages to using a clutch. The clutch imposes an additional load on the stepper motor and as a result the stepper motor must be larger and its drive circuit must operate at higher current levels. Furthermore, the belt tension is not easily adjusted using a slip clutch. Finally, changes in tension will occur due to clutch wear through use if the clutch is not regularly recalibrated by re-adjustment.

The prior art printers have typically been housed in enclosure structures that do not contribute to ease of assembly, repair, and reduction in manufacturing costs. In addition, the enclosure structures for the prior art thermal printers have not been optimally designed to accommodate typical operating environments and conditions.

For example, studies on thermal printers in the workplace have revealed that the thermal printers are often operated with the protective cover open to provide easy access for loading and changing the media and ribbon supply. As a result of operating the thermal printer with the protective plate open, the cover is often damaged or broken away from the printer body. Thus, it would be preferable for a thermal printer to have a housing structure in which the main cover can be easily removed.

State-of-the-art housing structures of thermal printers include numerous screws and Components. These housing structures were often formed from stamped and formed metal foil plates. The numerous screws and components in the housing structure required additional time during the original assembly as well as during repair of the thermal printer. Thus, it would be desirable to provide a thermal printer housing structure that can be quickly and easily assembled with as few screws as possible and conveniently disassembled when necessary.

The prior art thermal printers have another problem with regard to assembling and disassembling the sub-units. The various components or sub-units were often interrelated and interconnected. Thus, when assembling or repairing the prior art thermal printer, additional assembly or disassembly time was required. Furthermore, reconfiguring the prior art printers for a variety of printing operations was difficult due to the interconnections and linkages of the sub-units.

The prior art printers also have another problem with regard to the printing rollers used in the device. In a printer, a roller typically comprises a roller shaft defining a cylindrical roller surface. The roller shaft has shaft portions extending beyond each end and typically engage some form of ball bearing roller assembly. The roller assembly and printing roller are attached to a frame portion of the housing structure to support the printing roller in a desired position. Since a high degree of accuracy is required in the position of the platen, complicated snap ring washers and roller structures have been invented for mounting the platen in the housing structure. However, such complicated structures cause difficulties in manufacturing and repairing the printer. Thus, it would be desirable to provide a platen that simplifies the mounting of the platen in the housing structure.

As discussed above, the prior art thermal printing devices can be quite complicated and labor intensive in terms of assembly and disassembly procedures. The printhead assembly of the prior art thermal printers can also be quite complicated and require considerable effort to assemble or repair. One form of prior art printer employs a printhead assembly that rotates about an axis located between the platen frame and the housing structure. This arrangement provides only a single degree of freedom and therefore highly precise adjustment of the printhead relative to the platen and print media is difficult, if not impossible, to achieve. In other words, the frame structure supporting the platen is attached to the housing structure and provides a base for the printhead assembly. This arrangement of the printhead limits the movement of the printhead to only a tilting motion toward and away from the platen. Since the design of the print head is limited to one of the three degrees of freedom, high-precision fine adjustment of the print head relative to the print medium can be difficult, if not impossible, to achieve.

In addition, the layout of the print head assembly as discussed resulted in the adjustment parts of the print head assembly being difficult to access during a printing operation. Thus, the adjustments of the print head assembly must be performed by numerous iterations of printing a desired label and stopping the machine for adjustment. Such an iterative adjustment procedure can be quite time consuming and therefore inefficient.

Having discussed the problems related to the housing structure, platen and printhead assembly of the prior art thermal printers, we now turn to the media dispensing system or assembly and the problems encountered therewith in the prior art thermal printers. While such media dispensing assemblies serve their purpose, several exist with problems that are desirably addressed. Unassisted removal of a spent ribbon from the take-up spindle is difficult in that the ribbon is typically a very thin plastic material with a printing substance applied thereto. As the take-up spindle winds up the spent ribbon, the ribbon tends to wrap quite tightly around the outer surface of the spindle. In addition, the thin plastic material tends to be somewhat slippery and difficult to grasp when attempting to remove it from the spindle for disposal.

A state-of-the-art printer uses an empty ribbon core mounted on the spindle to collect the used printing ribbon. On the take-up spindle an empty core is attached and the used tape is wound around the empty core. When the used tape is discarded, the core is pulled off the spindle and the empty core with the used tape wound around it is discarded. This method is problematic in that an empty core must be available whenever a used tape is to be collected. If a core is not available, the tape could be wound around the spindle without the core, but removing the used tape from the spindle without the core is a very difficult task.

Another way to solve the problem of disposal of the spent tape is to provide a spindle having a wire form to provide a clearance between the spent tape and the spindle outer surface. In this regard, a U-shaped wire form is placed on the spindle with one leg of the U-shaped wire form extending into the spindle generally parallel to a central spindle axis and a second leg of the wire form lying on the surface of the spindle or slightly above the surface of the spindle. As tape is wound around the wire form on the spindle, a space is created between the spent tape and the spindle surface. When the spent tape is to be disposed of, the wire form is removed from the spindle and the spent tape is pulled axially from the spindle. However, this form of take-up spindle can be problematic in that it introduces loose parts and still requires the removal of a component relative to the spent tape. For example, the U-shaped wire form could be lost, which would create the problem of the spent tape would wrap around a bare spindle or a replacement wireform. In addition, removing the wireform from underneath the tightly wound spent tape can be somewhat difficult and is similar to removing spent tape from a spindle without a wireform.

A problem arises in the prior art printers with the reproducibility of the winding tension of the ribbon printing ribbon. This winding tension is critical to the smooth flow of the ribbon through the media path during the printing process. This requires that a relatively constant winding tension be maintained on the ribbon supply roll both during the unwinding process of printing and during the rewinding process discussed above. If sufficient tension is not maintained on the ribbon, or if slack occurs during rewinding, the ribbon may tend to smear or mark the media adjacent to it. In this regard, clutch mechanisms were invented for some prior art printers to provide winding tension on the printing ribbon. However, many clutch mechanisms were relatively complicated and required numerous parts for proper operation. The numerous parts accordingly resulted in additional costs as well as time-consuming and laborious assembly and repair. Thus, it would be desirable to provide a simplified coupling mechanism for use with a thermal printer.

Printers are often shipped overseas, which requires them to be able to operate on 240-volt power sources.

One state of the art way to accommodate both 120-volt and 240-volt operation with the same power supply structure is to use a jumper wire to select the desired operating voltage. It is also desirable to build and store printers in semi-finished form and then adapt the semi-finished units to either 120-volt or 240-volt operation prior to shipment.

US 2 533 973 describes a torque-limiting spring clutch device.

SUMMARY OF THE INVENTION

An on-demand printer according to the present invention as described in claim 1, wherein a preferred slip clutch assembly is described in claim 2.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention which are believed to be novel are set forth with particularity in the appended claims. The structure and mode of operation of the invention, together with further objects and advantages thereof, may best be understood by reference to the following detailed description taken in conjunction with the accompanying drawings. in which like reference numbers identify like elements and in which:

Fig. 1 is a perspective view of a preferred embodiment of an on-demand printer according to the invention;

Fig. 2 is an exploded view of the on-demand printer showing how some of the cover components have been removed;

Fig. 3 is a perspective view of the on-demand printer from another angle, showing some of the covers in an open position;

Fig. 4 is another exploded view of the on-demand printer illustrating various components;

Fig. 5 is yet another exploded view of the on-demand printer, illustrating the various components thereof;

Fig. 6 is a front plan view of the on-demand printer without the particular cover components in place;

Fig. 7 is a rear plan view of the on-demand printer without certain cover components in place;

Fig. 8 is a right-hand plan view of the on-demand printer with certain components of the cover removed;

Fig. 8A is a partial right plan view showing the wound ribbon and the supply roll media;

Fig. 8B is a view corresponding to Fig. 8A showing the wound media in the on-demand printer using a tail-loaded fan-fold media;

Fig. 8C is a view corresponding to Fig. 8A showing an arbitrary media rewinding device.

Fig. 9 is a left side plan view of the on-demand printer with certain cover components removed, and without a circuit board in place;

Fig. 10 is a left-hand side plan view of the on-demand printer corresponding to Fig. 9, but with a circuit board in place;

Fig. 11 is a partial exploded perspective view of certain components of the invention;

Fig. 12 is another partially exploded perspective view of certain components of the invention;

Fig. 13 is an exploded view of a roller assembly component according to the invention;

Fig. 14 is an exploded view of a pivot assembly according to the invention shown in a released position;

Fig. 15 is an exploded view of a pivot assembly according to the invention shown in a locked position;

Fig. 16 is an exploded perspective view of a media component according to the invention;

Fig. 17 is a perspective view of the medium sensor and guide plate assembly according to the invention;

Fig. 18 is an exploded perspective view of the medium sensor component according to the invention;

Fig. 19 illustrates some of the media types that can be used with the on-demand printer according to the invention;

Fig. 20 is a schematic electrical diagram of an electrical circuit connected to the medium sensor device according to the invention;

Fig. 21 is an exploded perspective view of a guide carriage component according to the invention;

Fig. 22 is a perspective view of the media rewind take-up spindle;

Fig. 23 is an exploded perspective view of a stepper motor component according to the invention;

Fig. 24 is a perspective view of a printhead assembly used in the on-demand printer;

Fig. 25 is a perspective view of a printhead assembly used in the on-demand printer;

Fig. 26 is an exploded perspective view of the print head assembly;

Fig. 27 is an exploded perspective view of an opened print head lever assembly of the on-demand printer;

Fig. 28 is a perspective view of a label receiving member of the sensor according to the invention;

Fig. 29 is an isolated perspective view of a tape take-up spindle and an associated drive mechanism;

Fig. 30 is an exploded view of a take-up spindle and the associated mechanism shown in Fig. 29;

Figures 30A and 30B are schematic representations of the operation of the take-up spindle;

Fig. 31 is an exploded perspective view of a spring clutch component according to the invention;

Fig. 31A is a perspective view showing the structure of the coupling boss;

Fig. 32A is a graph showing the relationship between speed and rotational force of a PMDC motor element of the tape take-up device component according to the invention;

Fig. 32B is a graph showing the relationship between motor current and torque;

Fig. 33A is a graph showing the relationship between motor speed and spindle tape winding radius;

Fig. 33B is a graph showing the relationship between tape force and tape winding radius of the spindle;

Fig. 34 is a block diagram showing the electrical interrelationships between the various components of the on-demand printer;

Fig. 35 to 51 are schematic circuit diagrams of various circuits used in the on-demand printer. The component meanings shown therein are for explanation purposes only.

Fig. 52 is a block diagram showing the process for printing a sticker;

Fig. 53 illustrates a typical label including typical label features;

Fig. 54 is a graphical representation of the sensor waveforms ;

Fig. 55 is an exploded perspective view of a power supply circuit taken out of a recess in the base of a printer and showing the means for converting the voltage setting of the printer; and

Fig. 56 provides additional information showing a divider inserted between a jumper wire to convert the voltage setting of the printer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the perspective view of Fig. 1, an on-demand printer 60 is shown. As shown in Fig. 1, the printer 60 is shown with a plurality of cover members in place to house the various operating components of the printer 60. The cover members include a control panel cover 62, a front panel 64, a folded down side panel 66, a fixed side panel 68, and a portion of a cutout of the base 70. Also shown in Fig. 1 is a hinge 72, which will be discussed in more detail below. The hinge 72 facilitates movement of the folded down side panel 66 upwardly away from the base segment 70 to access the various operating components of the printer 60.

Fig. 2 provides a view of the printer 60 in which the plates 64, 66, 68 have been exploded from the printer 60. The exploded view of Fig. 2 provides a front perspective view of the printer 60 to show the components. As shown in more detail in the following figures, a central support wall 74 is attached to the base 70. A central support wall provides structural support and a mounting surface for the various components of the printer 60. The folded down side plate 66 is removed from the central support wall 74 by disengaging the components of the hinge 72. The fixed side plate 68 is removed from the central support wall by removing a plurality of screws 76 that secure the fixed side plate 68 to the central support wall 74. The front plate 64 attaches to the base segment 70 via a front plate hinge 78, which is disclosed in more detail below.

Looking now at Figure 3, the printer 60 is seen from a rearward perspective showing the area covered by the folded down side panel 66. When the folded down side panel 66 is lifted from the base segment 70, several subassemblies and many components of the printer 60 are immediately visible. A printhead assembly 80 is shown and includes a printhead carrier 82 pivotally mounted to the central support wall 74 and a printhead assembly 84 mounted to the printhead carrier 82. The media dispensing assembly 86 includes a platen roller 88, a ribbon take-up spindle 90, and a ribbon feed spindle 92. The media dispensing assembly 86 includes additional components as discussed below. Referring to Figures 8A, 8B and 8C, a medium on which indicia are to be printed is fed to a medium supply stream 94 under the influence of the positively driven platen roller 88. 86 is associated with the ribbon feed spindle 92 and is fed to a ribbon feed stream 98 which generally follows the media feed stream 94. The ribbon 96 is drawn through the printer 60 under the influence of friction between the ribbon 96 and the media feed stream 94 and secondarily under the influence of the ribbon take-up spindle 90. The ribbon take-up spindle 90 and the new means for driving the spindle 90 are discussed in more detail below.

Referring again to Figure 3, a media sensor 100 is disposed in the media supply stream 94 to sense the position of the media passing through the media supply stream 94. A media guide 102 is provided with the media sensor 100 to properly position the media passing through the media supply stream 94 for proper sensing. The operation of the media sensor subassembly 100 of the present invention and the novel features thereof are discussed in more detail below.

The tilting assembly 103 is provided to position the print head assembly 84 close to the platen roller 88 for thermally printing imprints onto the media passing thereunder. The additional novel features of the tilting assembly 103 and the operation of the tilting assembly 103 with the print head carrier 82 are described in more detail below.

Fig. 4 provides a rear perspective view of the printer with the folded down side plate 66 and the fixed side plate 68 removed from the central support wall 74. Fig. 4 provides a View of the opposite side of the wall as shown in Figs. 2 and 3. While Figs. 2 and 3 show the components used in the actual transfer of prints to the media, the other side of the wall as shown in Fig. 4 provides the drive and circuitry for driving and controlling the printing components as shown in Figs. 2 and 3. A PMDC motor 104 is mounted on the central support wall 74 and drives the ribbon take-up spindle 90 through a gear assembly 106. The PMDC motor is coupled to the control circuitry 108. The PMDC motor is shown in the exploded view of Fig. 5 and in Figs. 9 and 29. Additional information on the operation of the PMDC motor 104 coupled to the control circuitry 108 is provided below.

In Fig. 4, a sub-axle gear and belt assembly 110 are shown. A sub-axle gear 112 is connected to a stepper motor 114 (see Figs. 8, 9 and 23) by a lay shaft 116. The drive motion generated by the stepper motor 114 and transmitted to the sub-axle gear 112 drives the belt 118 to also drive a roller gear 120 operatively connected to the print roller 88.

Fig. 5 provides an exploded view of the view shown in Fig. 4. Fig. 5 provides a view of the location of the projections or supports provided on the central support wall 74 through which support axles or drive shafts extend for supporting and driving the components on either side of the central support wall 74. For example, a medium hanger 122 and a stop bracket 124 attachable to the media hanger are shown separated from the central support wall 74. Further details and the novel features of the media hanger are disclosed in more detail below.

Figures 6 and 7 illustrate front and rear plan views of the printer as shown in Figure 4 (with the addition of the control circuitry 108 connected for driving).

Fig. 8, Fig. 9 and Fig. 10 illustrate side elevational views of the printer with the side covers 66, 68 removed from the central support wall 74. Figs. 8A, 8B and 8C illustrate various details concerning the passage of ribbon 96 and media 87 through the printer 60.

Referring now to Fig. 11, the components as shown in the perspective views of Figs. 2 through 4 have been removed from the printer 60, leaving essentially the central support wall 74 and the base segment 70. The components shown in Figs. 2 through 4 are suspended from the central support wall 74. A single reinforcement member 126 is attached to a protruding portion 127 of the central support wall 74. The reinforcement segment 126 provides additional structural support to minimize movement of the central support wall 74. The central support wall 74 is connected to the base segment by the foundation feet 128 (see Figs. 3 and 22) which engage below the foundation flanges 130.

As shown in Figure 8, one of the foundation flanges 130 has a slot 132 shaped to receive an upstanding pin 134 on the corresponding foundation foot portion 128. The engagement of the pin 134 in the slot 132 prevents the forward/backward movement of the central support wall 74 relative to the base 70. The engagement of the foundation foot 128 in the foundation flanges 130 provides a quick and convenient connection between the central support wall 74 and the base segment 70. The reinforcement segment connects to the central support wall 74 and the base 70 and also acts as a grounding rod for the entire printer. Thus, the reinforcement segment 126 is a metallic body to which the grounding straps are attached. A ground strap 136 connects the reinforcement segment 126 to a power supply circuit 138 contained in the base recess 140. The ground connection of the reinforcement segment 126 to the ground strap 136 is made via the power supply circuit 138 to the power cord.

Numerous structural supports and features have already been provided by directly molding such features into the central support wall to minimize the number of additional parts and the space used in the printer 60. For example, the helical teeth 142 for use with a slip clutch, details of which are provided below, are shaped to extend away from the central support wall 74. Similarly, to maximize the space within the volume defined by the housing structure 73, a bulge 144 has been formed in the central support wall to accommodate a portion of the PMDC motor 104 used to drive of the take-up spindle 90. Additional supports and other support structure were molded directly on either side of the central support wall 74. The aforementioned base recess 140 is shown more clearly in Fig. 12 with a bottom cover 146 removed to show the power supply circuit 148 fitting into the base recess 140 beneath a base foundation portion 148 of the base segment 170.

A bottom rib 150 of the central support wall 74 fits between a guide rail 152 extending upwardly from the base foundation 148 and a deck portion 154 of the base foundation 148. The lip 152 and deck 154 form a trough 156. A surface of the protruding portion 127 abuts an arm of a roller frame 158. When the bottom rib 150 is positioned in trough 156 and the foundation feet 128 engage the foundation flanges 130, a peg 160 extending from the protruding portion 127 engages a peg receiving device 162. Thus, the engagement of the central support walls 74 with the base segment 70 is essentially a screwless snap-fit operation. The exception to the screw-free design is the use of two screws on the drive side of the central support wall 74.

Referring to Fig. 23, the stepper motor 114 is mounted to the central support wall 74 through a motor mounting recess 164 as previously mentioned. The motor mounting recess has a recessed portion 166 defining an opening 168 through which the drive shaft 116 extends. The wall flanges 170 extend from the central support wall 74 into the recessed portion. 166. The motor flanges 172 on the stepper motor 114 engage the cooperating wall flanges 170 so that rotational rotation of the stepper motor 114 brings the stepper motor into the motor mounting recess 174. While Fig. 23 shows an exploded view of the stepper motor 114 in relation to the motor mounting recess 164, further views of the motor 114 mounted in the motor mounting recess 164 can be found in Figs. 3 and 9, and a view of the motor mounting recess 164 without a motor disposed therein can be found in Fig. 11. Figs. 9 and 11 show a nut post 174 formed in one of the wall flanges 170. The nut post receives a bolt or other fastener to provide additional security in holding the motor 114 in the motor mounting recess 164.

An additional feature provided in the central support wall 74 is the ability to quickly engage and release the media hanger 120. As shown in the enlarged, perspective, detailed exploded view of Figure 16, the media hanger 122 conveniently engages an opening 176 formed in a surface of the central support wall 74. A key segment 180 is formed on a mating end 162 of the media hanger 122. The key segment 180 includes a stem portion 184 that extends a distance from the mating end 182 and an enlarged portion 186 that extends generally perpendicular to the stem portion 184. The opening 176 is sized and dimensioned to receive the enlarged portion 186. A vertically oriented notch 188 is formed in of surface 178 in communication with opening 176. Vertically oriented notch 188 is sized and dimensioned to receive the key segment once enlarged portion 188 has been inserted into the opening. Movement of media hanger 122 downward causes stem 184 to engage vertically oriented notch 188. Further engagement is provided by press fit means 190 formed on a mating end 182 of media hanger 122 or surface 178 surrounding opening 176. As shown in Fig. 16, press fit means 190 includes press projections 192 formed on surface 178 and a mating rib 194 formed on mating end 182. A mating groove 196 is provided on surface 178 for receiving and mating rib 194. Engagement of stem 184 in notch 188 positions rib 194 for mating with mating groove 196. Press fit projections 192 provide an interference fit to further secure media hanger 122 to central support wall 74.

Referring to Figure 13, an enlarged, detailed, exploded perspective view of the platen roller 88 is provided. The platen roller 88 includes a roller shaft 198 defining a cylindrical roller surface 200. Typically, the roller surface is formed of a resilient, elastomeric material. In addition, the material used to form the roller shaft should provide a frictional force against the media pressed between the platen roller 88 and the printhead assembly 80 (see Figure 3). A central axis 202 extends longitudinally through the platen roller 88. The shaft portions 204 extend from each end of the roller shaft 198. The roller frame 158 extends upwardly from the deck 154 of the base foundation 148. The roller frame includes a first support arm 206 and a second support arm 208, a borehole 210 is formed in the first support arm 206 and a notch 212 is formed in the second support arm 208. Generally, the borehole 210 and notch 212 are approximately the same size. However, the notch 212 has an open end 214. Both the borehole 210 and notch 212 have similarly shaped mating surfaces, referred to herein as borehole mating surface 216 and notch mating surface 218.

Each of the shaft members 204 fits into a roller sleeve 220. The roller sleeves 220 provide smooth rotating surfaces for the shaft members 204. The sleeves prevent the need for ball bearing assemblies which complicate the elements and structure of the printer 60. Formed on an outer surface of the roller sleeves 220 are the sleeve mating surfaces 222. The sleeve mating surfaces 222 cooperatively fit the bore mating surface 216 and the notch mating surface 218 to prevent the roller sleeves 212 from rotating in the bore 210 and notch 212. The mating surfaces 212 and the sleeves 220 also have a stop surface 224 which limits the depth of engagement of the sleeve into the bore 210 and into the notch 212. The shims 226 are provided between the roller sleeves 220 and the abutting ends of the roller shaft 198.

ENG-DEU

The assembly of the pressure roller 88 with the roller frame 58 prevents the need for any screws to hold the pressure roller 88 in the roller frame 158. To assemble of pressure roller 58 and roller frame 158, the washers 226 and sleeves 220 are slid over the shaft portion 204. One end of the roller shaft 198 is positioned to slide the corresponding sleeve 220 into the bore 210 with the sleeve mating surfaces 222 aligned with the bore mating surfaces 216. Next, the opposite end of the roller shaft 198 is positioned so that the sleeve mating surfaces 222 are aligned with the notch mating surfaces 218. The roller sleeve 220 is inserted downward into notch 212.

14 and 15 provide an enlarged detailed view of the hinge 72 as previously introduced herein. The hinge 72 includes a pair of flexible arms 228 and a cylinder structure 230. As shown in FIG. 14, the pair of flexible arms in each hinge is connected to the central support wall 74 and the cylinder structure 230 is connected to the downwardly tilted side plate 66. Each of the flexible arms 228 includes a head 232 attached to the upper end of a stem 234, each of the heads and the pair of flexible arms having a facing surface 236. An extension 238 extends from the facing surfaces 236 of the pair of flexible arms 228, respectively. The pair of flexible arms 228 of each hinge 72 is formed along an upper ridge 240 of the central support wall 74. The arms are formed with a small gap 242 between a rear surface of each arm 244 and the ridge 240. The dimension of the gap 242 determines how far the arms 228 can bend outward from each other. In addition, a stop block 246 is formed between each pair of bendable arms 228 to limit the degree of inward to limit the movement of each arm. The gap 242 between the shaft 234 and the stop block 246 determines the degree of inward movement of the arms 228.

The cylinder structure 230 is secured to the pair of flexible arms 228 by positioning a cylinder bore 248 to engage a corresponding projection 238 formed on the surface 236 of the head portion 232. When the corresponding projection 238 engages the cylinder bore 248, pressure is applied to a central hinge axis 250, thereby forcing the engaged flexible arm 228 away from the second flexible arm 228 of the pair. By forcing the first flexible arm 228 away from the second flexible arm, the distance 252 between the arms 228 is increased. Next, a second end of the cylinder structure 230 is positioned on the projection 238 opposite the engaged projection 238. A downward force is applied to the cover 66 to cause the projection 238 to engage the corresponding cylinder bore 248.

The hinges can be used as a single set or in pairs as shown in Fig. 14. An additional feature of the hinge are the direction-determining bevels 254 formed on the extensions 238. When the cylinder structure 230 engages the pair of flexible arms 228, the assembled hinge 272 rotates about the central hinge axis 250. If too much force is applied to the hinge, the direction-determining bevels 254 facilitate the release of the cylinder structure 230 from the extensions 238. The direction-determining extensions can be either a bevel or a flat surface. As shown in Fig. 14, the direction-defining bevels 254 are angled inwardly toward the central hinge axis. An upper direction-defining bevel facilitates engagement of the extension 238 into the corresponding cylinder bore 248. The lower direction-defining bevel 254 facilitates disengagement from the cylinder bore 248 when oppositely directed forces are applied to the cover 66. The forces required to engage the extension 238 into the cylinder structure 230 define a direction of operation. Excessive forces or overload forces applied opposite to the direction of operation will cause the hinge to pop apart. The ability of the hinge to pop apart when excessive forces are applied substantially eliminates damage and the possibility of parts breaking. In addition, since thermal printers are often operated with the side plate 66 folded down for easy access to media 87 and ribbon 96, the hinges allow easy removal of the plate 66 from the housing structure 73.

The printhead assembly 80, as previously mentioned, will now be described in more detail with reference to Figs. 3 and 24-27. The printhead assembly 80, as shown in Fig. 3, has been disassembled in the enlarged perspective detail view shown in Fig. 26. As shown in Fig. 3, a fixed pivot axis 256 slides into a corresponding projection 258 formed on the central support wall 74. A pivot axis bracket 260 is attached to and extends away from the central support wall 74. A free end 262 of the pivot axis support bracket 260 carries a cooperating end of the rotary axis 256.

As better shown in Figure 26, a roll axis 264 is operatively connected to the pivot axis via a bore extending through a common hinge block 268 and a ring 270 which retains the roll axis 264 in the bore 266. The retaining members 272 are connected to the roll axis for engagement with a print head bracket 274. While the print head bracket 274 is retained under the retaining members 272, the bracket 274 is adjustable relative to the retaining members 272 by adjustment screws extending through the elongated holes 278. The print head assembly 84 is secured to a bottom surface 280 of the print head mounting bracket 274. As shown in Fig. 26, a ribbon strip plate 282 is attached to a front side 284 of the print head mounting bracket 274. The ribbon strip plate 282 is secured by screws that extend through elongated holes 286 formed in the strip plate. The elongated holes allow the strip plate to be adjusted up and down relative to the print head mounting bracket 274.

Referring to Figure 24, the fixed rotation axis 256, the roll axis 264, the printhead clamp 274 and the enclosed features collectively define a printhead carrier 288. The printhead carrier 288 positions the attached printhead 84 in a controlled manner immediately adjacent to media 87. The printhead carrier 288 allows for pitch, roll and yaw motion (as indicated by arrows 289, 291, 293, respectively) of the printhead 84. By providing the pitch, roll and yaw motion 289, 291, 293 the print head carrier 288 effectively provides free adjustment for the print head 84. The free adjustment of the print head 84 ensures that the print head 84 can be accurately adjusted. The pitch and roll movements 289, 291 of the print head are constantly changing, while the yaw movement is typically adjusted and then locked. The pitch movement 289 of the print head 84 is achieved by rotation of the fixed pivot axis 256 along a pivot axis access 290. The pitch movement 289 effectively moves the print head 84 parallel to and from the platen roller 88. The roll movement 291 of the print head 84 is achieved by rotation of the roll axis 264 in the bore 266. The yaw movement 293 is achieved by loosening the adjustment screws 276 and adjusting the print head mounting bracket 274 accordingly. In addition, since the printhead assembly 80 is supported by the central support wall 74, ribbon and media can be loaded or removed from the side of the printhead assembly 80. For example, media can be inserted for loading below the media guide 102 between the platen and the printhead 88, 84. If a jam occurs, access to the printhead assembly from the side is also available for easy clearance of the jam.

The printhead assembly 80, as discussed above, is also removable from printer 60 as a complete subassembly.

The yaw motion 293 of the print head 84 allows the print head to be adjusted and fine-tuned to achieve optimum print quality. The yaw motion 293 ensures that the print head and the series of inserted elements are aligned parallel to the platen roller 88. The adjustment screws 292 are provided on the front of the printer 60. The adjustment screws protrude through an adjustment projection 294 and contact a widening adjustment tab 296 extending downward from the printhead bracket 274. The adjustment screws 292 are tightened into the adjustment projections 294 and press against the widening adjustment tabs 296 to selectively and controllably fine-tune the side-to-side motion or yaw motion 293 of the printhead.

An important feature of the invention is that the adjustment of the yaw 293 of the print head 84 can be made during the printing process. In this regard, the print head position provides instantaneous results and feedback regarding the adjustment. The instantaneous feedback eliminates the need for the iterative steps common in prior art printers.

To adjust the print head 84, the adjustment screws 276 are loosened slightly so that a small degree of movement is possible between adjustment screws 276 and elongated holes 278 in the print head mounting bracket 274. A print operation is started and the alignment of the print is checked. An appropriate one of the two adjustment screws 292 is moved so that the widening adjustment tab and therefore the corresponding side of the print head mounting bracket 274 are moved. When a desired print head 84 alignment is achieved, this operation is stopped and the adjustment screws 276 are tightened to prevent further adjustment. The adjustment screws 292 are then removed from the adjustment bosses 294 and stored in a compartment in the housing structure to prevent further undesirable adjustment.

The tilt assembly 103 was previously mentioned and shown in Fig. 3. A further detailed description of the tilt assembly 103 is provided with further reference to Figs. 24, 25 and 27. Fig. 27 provided an exploded perspective view of the components comprising the tilt assembly 103. The tilt assembly engages and disengages the printhead 84 and media 87 by applying a force to the printhead mounting bracket 274 to tilt the printhead 84 against the platen roller 88. The tilt assembly includes a tilt arm 298 and a tilt plunger assembly 300. The tilt arm 298 also includes a shaft assembly 302 having a fitting portion 304 and a knob 308. The shaft assembly 302 is inserted through a hole 308 in the tilt arm 298, and the fitting portion 304 positively engages a correspondingly shaped portion in hole 308. The knob 306 is provided to provide additional ease of operation and transmission of mechanical force in the operation of the tilt assembly 103. One end of the shaft assembly 302 is connected to the central support wall 74 generally parallel to the print head 84.

A pair of plunger tubes 310 are provided on the tilt arm 298 at spaced locations and oriented generally perpendicular to the shaft assembly 302. The tilt plunger assembly 300 is mounted in a cavity 312 of the plunger tube 310. The tilt plunger assembly 300 includes a plunger head 314 tilt assembly 316 and an adjustment member 318. The plunger head 314 is retained in the plunger tube 310 such that a rounded tip portion 320 extends at a bottom portion of the plunger tube 310. The opening to the plunger tube bottom cavity 312 has an extent approximately equal to the diameter of the plunger head and less than a retaining ring 322 formed on the head spaced from the rounded tip portion 320. The tilt assembly 316 presses against an end portion 324 of the plunger 314. The adjustment member 318 is essentially a threaded wing screw which engages the top portion of the cavity 312 of the plunger tube 310. The adjustment member 318 is rotated to increase or decrease the tilt forces against the plunger head 314.

Referring to Figures 24 and 27, the tilt assembly 103 is shown in use with printer 60. When a user adjusts the tilt assembly 103 to bring printhead 84 into contact with media 87, the user grasps the knob 306 and rotates it along a tilt axis 326 (shown by arrow 328) to move the rounded tip portion 320 into engagement with the printhead support bracket 274. By rotating the tilt arm 298 by rotating the shaft assembly 302, the tilt arm describes an arc that eventually forces the rounded tip portions 320 of the plunger heads 314 into contact with the printhead support bracket 274. Since the plunger heads 314 are held inclined into the plunger tube 310, the sliding contact against the print head support bracket 274 presses the plunger head 314 against the 316 upward into the plunger tube 310. The compressive forces exerted by the tilter 103 on the print head assembly maintain a desired force on the print head 84 and press it against the platen roller 88. The above-mentioned desired force can be adjusted by adjusting the adjustment part 318 to increase or decrease the tilting force of the tilter 316 against the plunger head 314.

The invention also includes a sensor device 330 for indicating whether the print head 84 is in contact with or disengaged from the media or the roller 87, 88. The contact of the print head 84 is directly dependent on the position of the tilting device 103, since it is the tilting device that brings the print head 84 into contact or disengages it. Thus, the rotational position of the shaft assembly 302 is used to indicate the condition of the print head 84. Referring to Fig. 25, the sensor device 330 includes an optical sensor 332 and a sensor connection 334 that is directly connected to the shaft assembly 302 of the tilting device 103. The optical sensor 332 includes an optical transmitter 336 and an optical receiver 338. The optical transmitter 336 emits a beam of light that is received by the optical receiver 338. The link 334 extends from shaft 302 and rotates along a path 340 that runs between optical transmitter and receiver 336, 338. It should be noted that sensors other than purely optical sensors could be used in this configuration.

When using this particular embodiment of the invention, the connection 334 for breaking the beam path between optical transmitter and receiver 336, 338 when tilting means 103 is engaged with printhead means 84. When the tilting means is disengaged, link 334 rotates upward along path 340 out of the beam path, thereby allowing the optical circuit to be reestablished. Of course, the signals could be reversed so that the beam between transmitter and receiver 336, 338 is open when tilting means 103 is engaged with printhead means 84, and the beam is broken when tilting means 103 is disengaged from printhead means 84. Since optical sensor 332 is connected directly to a circuit board 342 which includes control circuit means 108, additional wiring in connectors or connections is not required. The signals from the optical sensor 332 are received and processed by the control circuit 108 and can be used to prevent further operation until a previously set printhead condition is reached.

Fig. 28 provides an enlarged perspective view of the front of the printer showing an opening 344 defined between the tape strip plate 282 and a serrated tear-off edge 346. In the view shown in Fig. 28, the media and tape have been removed for clarity of description of the components shown therein. If media and tape 87, 96 are shown, the media and tape 87, 96 would pass through the opening 344. The tape would pass up over the tape strip plate 282 and then wrap around the tape take-up spindle 90. The media 87 would exit the opening and travel along a path, which is defined by a label take-up sensor 348. The label take-up sensor 348 includes a transmitter portion 350 and a receiver portion 352. The transmitter portion 350 transmits a signal to the receiver portion 352 and creates a sensor barrier therebetween. As media exits the opening 344, it exits outward and crosses the sensor barrier. Upon crossing the sensor barrier, the label take-up sensor 348 senses the presence of the media and transmits an appropriate signal to the control circuitry 108. Upon removal of a portion of the media 87, the sensor barrier is no longer crossed and another signal is transmitted to the control circuitry 108. The label take-up sensor 348 and the control signals generated thereby are coupled to the media dispensing device 86 to facilitate the controlled movement of media 87 and ribbon 96 relative to the print head 84.

The movement of the ribbon 96 is achieved by positively driving the ribbon take-up spindle 90 with the PMDC motor 104. The novel features of the structure and function of the PMDC motor are provided in more detail in a separate part of this detailed description. The PMDC motor, however, provides the positive driving forces through the bevel gear assembly 106. A shaft 354 engaging the bevel gear assembly drives the ribbon take-up spindle 90. The perspective views of the ribbon take-up spindle 90 and PMDC motor with the central support wall 74 removed are illustrated. To show the location and mounting of the ribbon take-up spindle 90 and PMDC motor in the printer 60, reference is made to Figures 2-5.

As shown in Fig. 29 and with further reference to Fig. 30, the tape take-up spindle 90 has a cylindrical outer surface 356 with at least one upstanding bore 358 formed therein. As shown in Fig. 29, two diametrically disposed projecting openings 358 are provided on the spindle surface 356. The openings 358 extend longitudinally parallel to the spindle central axis 360 and define slots through which the projecting segments 362 project. The projecting segments 362 also extend longitudinally and define blades that project through a corresponding slot 358.

As shown in Fig. 30, spindle 90 is formed from two body halves 364. A portion of each slot 358 is formed in each body half 364. Four engagement pins 366 lock the two halves 364, 364 together and form a unitary spindle body. In addition, blades 362 are formed with guide openings 368 that fit into engagement pins 366. When blades 362 are fitted onto engagement pins 366, the blades are constrained to movement that is generally radial and perpendicular to the central spindle axis 360 and is limited by the size of guide openings 368.

As shown in the exploded view of Fig. 30, the spindle 90 also includes the tilting device 370 and the device 372 for retracting the blades 362. The tilting device 370 tilts and directs the blades 362 outwardly through the corresponding slots 358 in a controlled manner. The retracting device 372 can be actuated to Tilt device 370 to be compressed in a controlled manner to retract blades 362 into spindle 90.

When the blades 362 are extended through the slots 358 and spent ribbon 396 is wound around the spindle 90, a space is defined in part by an extension 374 between a face 376 of the blades 362 and the surface 356 of the spindle 90. In other words, when the spent ribbon 96 is wound around the spindle 90, a space is formed between the ribbon winding over the face 376 of the blade 362 to the point where the ribbon is rewound around the surface 356 of the spindle 90. When the spent ribbon 96 is to be removed from the spindle 90, a retractor button 378 is pushed inward along the central axis 360 to actuate the retractor 372. As the transverse tension on the blades 362 is released, the volume defined by the space between the blade and the spent tape is expanded over the entire circumference and surface 356 of the spindle 90. The additional space between the spent tape and the surface 356 of the spindle 90 allows the spent tape to be easily removed from the spindle without the spindle pushing through the spent tape and without using loose components such as wire forms that have been used in prior art designs.

The retractor 372 operates under the influence of the inclination device 370 such that the inclination device inclines a retractor body axially coincident with the central spindle axis 360. The retractor body 380 is operatively held between the two spindle halves 364, 364. The retractor body 380 includes two tines 382 having the shank ramps 384 formed on the outwardly facing surfaces thereof. The blades include co-formed blade ramps 386 which move along and engage the shank ramps 384.

30A and 30B provide additional clarifying explanations to show how the retractor 372 and the tilter 370 function to effect the movement of the blades 362. As shown in the schematic of FIG. 30A, the blades 362 are extended outwardly through the slots 358. The extended blade condition as shown in FIG. 30A is caused by the tilter 370 being held between shaft 354 and retractor body 380 and transmitting expansion forces from the tilter 370 to the retractor body 380. Since the shaft 354 is stationary and does not move axially along the central spindle axis 360 and since the retractor body 380 is movably supported within the spindle, the inclination device 370 translates the retractor body 380 axially along the central spindle axis 360. As the body is translated along the central spindle axis 360, the blade slopes 386 slide upward along the adjacent surfaces of the shaft slopes 384 and rise to a highest point of each shaft slope 384. When the highest points 388 of the shaft slopes 384 are adjacent to the corresponding highest points 390 of the blades 362, the blades are fully extended and retract under the influence of of the tape tightly wrapped over the surface 376 of the blades 362. In addition, axial movement of the retractor body 380 along the central spindle axis 360 is prevented by a stop rim 392 adjacent an inner surface 394 of the spindle halves 364. In this regard, the tilting means 370 may be selected to continue to exert forces on the retractor body 380 when the blades are fully extended. The additional forces generated by the tilting means 370 further insure that the blades remain in the extended position when not selected to be retracted.

Referring to Fig. 30B, the schematic shows the retraction of the blades when the retractor body 380 is manually displaced along the central spindle axis 360. When the retractor body 380 is manually displaced along the central spindle axis 360, the inclination device 370 is compressed between the shaft 354 and the body 380. By releasing the inclination force, the blade slopes 386 can be moved downward along the corresponding shaft slope 384, thereby allowing inward movement of the blades 362. It should be noted that in both Figs. 30A and 30B, the blades only move radially outward along the guide openings 368. The engagement of the engagement pins 366 in the blades 362 as well as the limited size of the slots 358 prevents displacement parallel to the central spindle axis 360.

Control of the ink ribbon 96 in the printer 60 is further facilitated by a slip clutch 396 operatively connected to the ribbon feed spindle 92. The ribbon feed spindle 92 has a shaft 398 extending through the central support wall 74. A clutch axis 400 extends longitudinally along the spindle shaft 398. The slip clutch 396 includes a series of inclined teeth 142 spaced about the spindle shaft 398, a spiral torsion spring 402 coaxially slipped over the spindle shaft 398, and a clutch collar 404 housing a portion of the spiral spring 402 and fixedly connected to the spindle shaft 398.

In assembling the slip clutch assembly, the spindle shaft 398 is inserted through the central support wall 74 and is pivotally secured with a retaining collar 406. The spiral torsion spring 402 is inserted into a spring hole 408 in the clutch collar 404, and the combination of torsion spring 402 and clutch collar 404 is placed over the spindle shaft 398. The clutch collar 404 is secured at one end by an adjustment screw 412. A straight end portion 414 of the helical torsion spring 402 extends from the clutch collar 404 and radially from the spring 402 to engage the inclined peripheral surfaces 416 and the vertical walls 418 adjacent the inclined surface 416.

The spiral torsion spring 402 is selected to have a calculated interference fit between an outer diameter of the spring 420 and an inner diameter 422 of the spring hole 408 in the coupling sleeve 404. The The amount of diametrical compression is directly proportional to the amount of resistance force provided by the spring 402. The coefficient of friction of the spring 402 and the sleeve 404 and the length of engagement are excluded from the calculations for the sliding torque for practical reasons. This allows greater flexibility in the design with regard to geometry and the choice of material for the spiral spring 402 and the coupling sleeve 404.

The collar 404 is attached to the shaft 398 so that they rotate as a unit. When the shaft 398 rotates (as indicated by arrow 424), i.e. so that the driving force on the take-up spindle 90 applies tension to the tape on the unwind spindle 92, the spring 402 and collar 404 rotate together until the extended end of the spring engages between a vertical wall 418 of a corresponding beveled tooth 142. Under the influence of the rotation 424, the spring 402 is rotated or compressed by rotating in the direction of its manufactured coil. This rotation effectively reduces the outside diameter 420 of the spring 402 until it reaches a point where an outer surface 426 of the spring slides against an inner surface 428 of the spring hole 408. A calculated amount of shaft rotation, i.e., winding in the spring, is required before the correct slip situation is achieved. As the shaft 398 continues to be driven in the direction of rotation 424, the spring 402 continues to slide and maintains a constant tension on the sleeve and a constant amount of winding.

When the driving force is no longer present or decreases in the direction of rotation 424, the memory in the spring 402 causes it to rotate in an opposite direction its manufactured coil through an angle corresponding to the slip coil. This opposite action or winding of the spring 402 is accompanied by a return to its original manufactured diameter 420. When the spring diameter 420 reaches a predetermined dimension, the outer surface 426 of the spring 402 binds to an inner surface 428 of the spring hole 408 in the coupling sleeve 404 and causes the sleeve 404, and thus the shaft 398, to rotate with it.

Due to the fact that the spring outer diameter 420 increases when it is rotated in the opposite direction of its coil (opposite the direction of rotation 424 as shown in Fig. 31), spring damage can occur if the shaft 398 and collar 404 are pushed in the opposite direction, holding the extended end piece 414 in a fixed position. Since it is likely that the user will want to reverse the tape feed spindle 92 attached to the shaft 398 at some point, particularly when loading a new roll of tape, the inclined surfaces 416 are provided to allow the extended end piece 414 to rotate freely backwards while still engaging the spring hole 408 of the coupling collar 404. The arrangement of the beveled teeth spaced circumferentially about the clutch axis 400 provides a ratchet-like feature in which the extended end piece 414 is held against a vertical wall 418 forward in the drive direction 424, but is allowed to ride up along the beveled surface 416 and over a bevel 142 indefinitely in a direction 430 opposite to the direction of drive rotation 424.

The slip clutch 396 provides a simple and inexpensive means for applying winding tension to the ribbon feed spindle 92 in the printer 60 to reduce wrinkles in the ribbon 92 moving through the ribbon feed stream 98. In addition, the slip clutch 396 also provides for rewinding of the ribbon 96 and ribbon feed stream 98 when the printer 60 retracts or rewinds the media 87 into storage to reposition a leading edge of the media during printing or after removal of a portion of the printed media. This rewind feature is very important for thermal transfer printing because it maintains winding tension on the ribbon 96 through the rewind cycle. If the ribbon 96 is not maintained under tension when the printer 60 is accelerating forward in a normal printing direction, the inertia of the ribbon roll can cause the ribbon 96 to move in a jerky manner, which can result in smearing on the portion of the media being printed on. In addition, the jerking action described above can cause wrinkles in the ribbon and therefore create inconsistencies in print quality. These inconsistencies can be extremely detrimental when printing high resolution print such as knitting codes or a very small character.

SELF-CORRECTION SYSTEM OF THE TAPE REWINDING SPINDLE

Another problem that occurs with thermal transfer on-demand printers is that the tension on the ribbon does not remain consistent during printing. This decrease in tension causes the ribbon to tend to wrinkle during printing, which can lead to The resulting sticker may contain defects such as inconsistencies in print quality.

The reason for this is that when the used ribbon is wound onto the take-up spindle, the radius of the take-up spindle increases as the printer continues printing. As the radius of the ribbon take-up spindle increases, the force, i.e. the tension applied to the ribbon, decreases unless the ribbon take-up spindle torque is increased. This effect is given by the following equation:

Belt force = (spindle torque)/(spindle radius)

Thus, to minimize this problem, the tape take-up spindle torque must be increased as the tape take-up spindle radius increases.

This problem is minimized in the invention by employing a self-correcting system that utilizes the characteristics of a permanent magnet direct current (PMDC) motor when a constant voltage is applied to its outputs. As shown in Fig. 29, the self-correcting system generally comprises a PMDC motor, a gear assembly including a gearbox and a tape take-up spindle.

The shaft of the take-up spindle, as described here, is connected to the center of the gear box by a suitable device. For example, the shaft can snap into a hole in the gear reduction and be held in place with a screw. The two components form an interference fit. The gear reducer is circular in shape and has an outer edge that is tapered. The PMDC motor is connected to a suitable power source through the printed circuit board ("PCB"). The PCB includes suitable microprocessors to perform the printer functions as described herein. The PMDC motor may be connected to a standard linear regulator, which may be included in the PCB, for controlling the amount of voltage applied to the PMDC motor. A tapered flange extending from one end of the PMDC motor is in contact with the tapered outer edge of the circular gear reducer. The tapered end of the PMDC motor and the tapered outer edge of the gear reducer are connected to one another forming an interference fit between the components. In operation, the PMDC motor drives the gear reducer, which in turn rotates the take-up spindle. Thus, the spent ribbon is wound onto the take-up spindle.

When the PMDC motor has a constant voltage applied to its outputs, the PMDC motor obeys the characteristics of this speed-torque curve shown in the graph of Fig. 32A. As can be seen from the graph, as the speed of the PMDC motor decreases, its torque output increases. This is useful in a belt tensioning system because the system is self-correcting, as described in more detail below.

If the printer prints at a constant print speed, as the diameter of the take-up spindle increases, its take-up speed decreases. This decrease in take-up speed results in the speed of the PMDC motor decreases proportionally. As the EM counterforce generated by the PMDC motor decreases, resulting in an increase in current flow in the PMDC motor. As the current flow increases (and the speed decreases), the PMDC obeys its speed-torque curve and thus its torque output increases. This increase in torque causes the force on the tape, the tension, to increase. Therefore, the system corrects itself and the tape tension exhibits less variation due to the increase in the tape take-up spindle diameter.

In the preferred embodiment, a low gear reduction is used. As shown in Figure 33A, the graph represents a model system employing a gear reduction of 5 to 1 from PMDC motor to tape take-up spindle. As can be seen, the tape take-up spindle radius varies from 3.048 cm to 5.334 cm. As shown in the graph, the PMDC motor speed decreases as the take-up spindle radius increases. Thus, the PMDC motor obeys its speed-torque curve as shown in Figure 32A and increases its torque output. If this system is used with a tape running at a linear speed of 5.08 cm per second, an effective, self-correcting tape tension control system can be established. It is to be understood, however, that other low gear reductions can be used with the invention.

In Fig. 33B, a graph of tape tension as a function of take-up spindle radius is shown comparing a non-correcting system and a self-correcting system. The non-correcting system explained could be This can be achieved using a slip clutch, which is well known in the art. As shown in the graph, the non-corrective system starts with an empty take-up spindle and a tape tension of approximately 390 g as shown by the dashed line. With the tape take-up spindle full, the tape force decreases to 240 g due to the increase in the tape take-up spindle radius.

When the self-correcting system is used, as shown by the solid line, the tape tension starts at about 390 g with an empty spindle and decreases to about 340 g when the tape take-up spindle is full. Thus, a significant improvement is achieved by the application of the invention.

If the user wants to run the printer faster or slower, the user enters a new print speed. When the print speed is changed, the PMDC motor operates on a different part of its speed-torque curve. Therefore, it is necessary for the drive circuit to receive information about the printer's operating speed so that the printer can change the operating voltage of the PMDC motor.

Another advantage of using a PMDC motor is that it reduces the load on the stepper motor. This allows a smaller stepper motor to be used to drive the rest of the printer's components.

Another feature of the invention is that the printer can be used with different widths of the ribbon and still maintain a relatively constant ribbon tension. With thermal transfer printers, it is often desirable to use different width ribbons depending on the width of the label being printed in order to avoid ribbon waste and therefore minimize costs. For example, if a 2-inch wide label is fed into a thermal printer, using a 6-inch wide ribbon in the printer would not be cost-effective. Therefore, a narrower ribbon is used. ·

When using a narrower ribbon, it is advisable to reduce the torque of the ribbon take-up spindle so that the ribbon tension is maintained at a safe level. If this is not done, ribbon tearing and stretching can occur. For example, if the thermal transfer printer user adjusts the spindle torque to apply an appropriate amount of force to a 6" wide ribbon, and the user has a 3" wide ribbon loaded on the printer, the ribbon's tensile elongation would increase by a factor of 2. Thus, the ribbon would easily tear or stretch.

In an alternative embodiment of the invention, the PMDC motor can be driven by a pulse width modulation (PWM) control loop as shown in Figure 35 to produce a pulse width modulated signal. The PWM control loop operates cooler than a standard linear controller because it is more effective when driving an inductive load such as a motor. This PWM control loop allows the user to select a desired torque for the PMDC motor. When the circuit is operating as described in more detail below, the speed/torque characteristics of the PMDC motor remain relatively constant, even with large changes in the motor's line voltage ("VHEAD").

In thermal transfer printers, the electronics typically operate at +5 vdc, with the exception of the thermal print head, which typically operates between 5-40 vdc to heat the print head's thermocouples. During the thermal print head manufacturing process, variations in element resistance can occur. This requires the printer to change the voltage applied to the print head to compensate for this change in resistance. If the voltage is not changed to compensate for the variations in element resistance, print quality will suffer.

This PWM control loop enables the PMDC motor to have a relatively constant average voltage applied to the PMDC output regardless of the line voltage. This allows the PMDC motor to obey its speed-torque curve and improves the variation in belt tension as described above.

The PWM control circuit can be integrated into the PCB and is also connected to the PMDC motor by suitable wiring. The PMDC motor drives the spindle in the same way as described above.

The circuit shown in Fig. 35 consists of a NE556 IC timer. The NE556 IC timer consists of two NE555 timers in a single assembly. One of the NE555 timers is configured as an unstable multivibrator. In the preferred embodiment, the unstable multivibrator is designed to output a square wave at 5.9 KHz with a duty cycle of approximately 81%. The output of the unstable multivibrator is fed into the other NE555 timer, which is configured as a monostable multivibrator. As a negative transition occurs on the unstable multivibrator, the monostable multivibrator is excited and emits a pulse of a duration given by the following equation:

Pulse width = -(R) (C) (ln(1-3,333/VHEAD))

where: VHEAD = mains voltage of the PMDC motor;

R = timing resistor, monostable;

C = timing capacitor, monostable; and

3.333 = Turn-off threshold for the NE555 monostable multivibrator.

Resistor and capacitor, which determine the time constant for the monostable multivibrator, are connected to the PMDC motor line voltage in a manner shown in Figure... :

R = (RV3 + R31) in the previous equation

and C = (C26) in the previous equation.

The output pulse of the monostable multivibrator is fed into the gate of a MOSFET which pulses the PMDC motor with the voltage present at VHEAD. If a +5 VDC signal is applied to the RIBEN (Band Voltage Enable) line from the microprocessor, in the preferred embodiment this signal activates the monostable multivibrator which in turn turns on the PMDC motor. Similarly, applying a zero voltage signal on the RIBEN line, the monostable multivibrator is switched off, which in turn turns off the PMDC motor. The circuit pulses the PMDC motor at a frequency that is high enough, about 6 KHz, so that the print quality is not affected. If slow pulse speeds are applied to the PMDC motor, alternating dark and light bands appear on the media. This is due to the vibration of the PMDC motor, which causes the media and ribbon to vibrate.

In the preferred embodiment, the elements in the circuit assume the following values:

Of course, other values can be used depending on the application.

This circuit allows the tape take-up spindle torque to remain relatively constant while being independent of the PMDC motor's line voltage. If the PMDC motor line voltage VHEAD, the circuit compensates to allow the speed/torque characteristics of the PMDC motor to remain relatively constant. An additional benefit is that the circuit pulses the PMDC motor to limit the power consumption of the drive circuit. This results in the circuit being very efficient and little heat being generated by the electronics.

As can be seen from the foregoing, as the PMDC motor line voltage VHEA increases in value, the pulse width decreases in length, keeping the average voltage present at the PMDC motor outputs relatively constant. Similarly, as the value of VHEAD decreases, the pulse width for the PMDC motor increases in length, causing the average voltage to remain constant.

Since the PMDC motor must be able to operate near a blockage, in order to increase the life of the brushes in the PMDC motor, the voltage must be maintained at a level below its set operating voltage to limit the current to a safe level. In other words, the maximum current draw to the PMDC motor is limited by lowering its operating voltage. In the invention, the PMDC motor is operated with a DC voltage below its specified operating voltage and thus the PMDC motor cannot start to rotate. Therefore, it is convenient to pulse the PMDC motor with narrow pulses of an amplitude equal to the operating voltage of the PMDC motor to improve the starting characteristics of the PMDC motor.

The average voltage pulsed to the PMDC motor must be an equivalent DC voltage that would limit the PMDC current draw to a safe operating level at the motor speeds at which the invention operates. The PWM control loop described here pulses a PMDC motor at a peak amplitude determined by the voltage present at VHEAD. If VHEAD either increases or decreases, the circuit compensates and increases or decreases the pulse width of the voltage applied to the PMDC motor. The pulse width changes to maintain a relatively constant average voltage at the PMDC motor outputs.

The circuit also allows the tape tension to be adjusted by a potentiometer RV3 to control the tape take-up spindle torque and finally the tape tension to compensate for the variations in tape load due to the change in tape widths. By using the potentiometer, the tape tension can be slightly lowered to avoid damaging the tape. This is an improvement over the mechanical clutches of the prior art which are very difficult to adjust.

When the potentiometer is adjusted, the duty cycles of the pulses controlling the PMDC motor are either increased or decreased to change the speed vs. torque characteristics of the PMDC motor. The circuit continues to adjust the duty cycle according to the motor line voltage without taking into account the state of the potentiometer setting. For example, if the motor line voltage changes, the circuit automatically adjusts the duty cycle so that the average voltage applied to the PMDC motor output remains relatively constant.

The tape tension could also be adjusted by software control to control the tape take-up spindle torque and finally to compensate the tape tension for variations in tape loading due to the change in tape widths. The software and/or hardware could be modified to change the resistance values for R31 to change the RC time constant on the monostable multivibrator. This results in a change in the pulse width for the motor. Using software control, the tape tension can be easily changed to obtain the optimum tape tension. This represents a further improvement over the state of the art mechanical clutches.

The printer of the invention could also be modified to operate at a different print speed if the effective voltage on the PMDC motor varied accordingly. For example, if the motor voltage was increased when the printer changed print speeds from 2 inches per second to 6 inches per second, there would be less variation in ribbon tension due to an increase in printer speed. This could be achieved by having a microprocessor switch with different resistance values for R31. This would increase or decrease the pulse width voltage at the motor outputs.

Another feature of the invention is that the life of the PMDC motor is increased. The three main factors that control the life of a PMDC motor are: brush wear, armature life and bearing wear. Both brush wear and bearing life are dependent on the number of rotations the PMDC motor makes. If the number of rotations the PMDC motor has to make decreases in any way, the PMDC motor life could be increased.

If the PMDC motor is forced to run at a lower speed, i.e. near a stall, the counter EM force generated by the PMDC motor decreases, causing an increase in current flow to the PMDC motor. If the current flow is too large, damage to the armature windings can occur. If the current flowing through the PMDC motor has been limited by applying a lower than normal operating voltage to the PMDC motor, the armature winding life is increased because excessive current does not flow through the PMDC motor.

In the preferred embodiment, as described here, a low gear ratio is used. This allows the motor to operate more slowly than if a very high gear ratio is used. In addition, since the tape take-up spindle has a large diameter, the angular velocity at which the tape take-up motor would have to rotate is much smaller. Thus, the PMDC motor does not have to rotate as fast as if a small diameter tape take-up spindle is used. Therefore, the life of the PMDC motor is increased.

The PMDC motor also has the ability to be turned off by software control, and thus the PMDC motor is not in a stalled state. If the PMDC motor is in a stalled state for any period of time, for example when the printer is turned on without printing, the armature winding tends to become hot, reducing its useful life, even though the current flowing through the armature winding has been limited to a safe value by the operating voltage.

Another feature of the invention is that the on-demand printer described in this patent is capable of printing in thermal transfer mode, which requires ribbon. This on-demand printer is also capable of printing in direct thermal mode, which does not require ribbon. In the prior art, where the ribbon take-up spindles were driven by mechanical clutches, no easy way existed to disable the ribbon take-up spindle and stop rotation when the ribbon take-up spindle was not being used in a direct thermal application.

When the PMDC motor is used to drive the tape take-up spindle, it can be easily switched off in direct thermal applications by using the "RIBEN" line described in this invention.

It is desirable that the tape take-up spindle be deactivated when not in use, as it consumes energy and causes unnecessary wear on the tape take-up part.

Another feature of the invention is that the printer is capable of reversing the flow of the media and the printing direction of the ribbon as described herein. This feature is called rewind.

When a rewind process occurs, it is essential that the force required to pull the ribbon in the opposite direction is not too great. If the force required is too great, the ribbon will not be able to unwind from the ribbon take-up spindle because the printer components that control the rewind process may not have the ability to transmit the required amount of ribbon force in the rewind direction to unwind the ribbon. This is done in two ways.

First, the gear reduction from the tape tension motor to the feed spindle is minimized. This is done to limit the reflected moment of inertia from the PMDC motor to the tape take-up spindle. The reflected moment of inertia is given by the following equation:

reflected moment of inertia = motor moment of inertia · gear reduction²

The reflected moment of inertia increases by the square of the gear ratio. Thus, it is essential that the gear ratio is kept to a minimum to avoid an increase in the tape take-up spindle moment of inertia. If the moment of inertia reflected to the tape take-up spindle is too high, the initial force to unwind the tape from the tape take-up spindle will be too large.

Second, the PMDC motor can be driven by the PWM control loop, which has the ability to lock the PMDC motor from a control signal, as described above. This prevents the PMDC motor from transmitting torque to the tape take-up spindle. The PMDC motor must be locked to allow the tape to rewind at the same speed as the media. If the PMDC motor is not locked, the tape will not rewind and cause smearing of the tape on the media. Thus, since the PMDC motor can be locked, the amount of force required to rewind the tape is minimized.

ENG-DEU

In another aspect of the invention, a media sensor 100 is provided for monitoring and adjusting the position of the media in the on-demand printer, thereby ensuring accurate printing operations. In Fig. 17, the media sensor 100 is shown in operative connection with a media guide 102 which guides the path of media past the media sensor 100, thereby enabling the sensor 100 to perform its intended function. In Fig. 18, the media sensor 100 and media guide 102 are shown in disassembled form, and the remaining components of the printer 60 are shown in exploded form. Close inspection of Fig. 18 shows that the media sensor 100 includes a housing 484 having a cover 484 and a base 486 for enclosing a media sensor circuit board 468. Cover 484, base 486 and circuit board 488 each have a corresponding slot 490 formed therein that allows the media to pass through the media sensor 100.

As background information, it should be noted that the on-demand printer must be designed to print individual pressure-sensitive stickers 506 and tickets or labels 508 as shown in Figure 19. The pressure-sensitive sticker medium 510 is typically in the form of a continuous web having a paper backing 512 made of wax or silicone impregnated paper with a thickness range of between 0.05 mm and 0.20 mm and a plurality of stickers 506 made of paper, polyester, synthetic paper or similar material of similar thickness, removably attached with a rubber or acrylic adhesive. The successive labels 506 are separated by a label gap 514, which is typically 0.125" wide. The web may be fed from a roll or, alternatively, from a zig-zag fold. The tickets or labels 508 may similarly be presented in a continuous web 516, with the individual tickets or labels 508 defined by a printed visible indicia or by the punched holes 518 or notches 520. The ticket or label 516 media typically ranges in thickness from 0.18 mm to 0.46 mm.

A media sensor 100 is generally used to align a printed image with a leading edge of each sticker 506, ticket or label 508. As previously stated, optical media sensor 100 generally includes an illumination source such as an LED 492 and a photodetector such as a phototransistor or photodiode 494. Illumination source 492 and photodetector 494 typically, but without limitation, operate at 940 nm, an infrared wavelength.

In a preferred embodiment, the circuit board 488 includes an illumination source in the form of one or more light emitting diodes (LEDs) 492, such as an LED IR 950 NN shown in (Fig. 20) and located below the slot 490. The board 488 also preferably includes a photodetector device located above the slot 490, including a phototransistor or photodiode 494 (Fig. 20) adjustably coupled to the board 488 via a support 496 and a wire ribbon 498. The diode support is then connected to an adjustment arm 500 which is accessible through an opening 502 in the base 486 and which rides along a track 504 provided at the bottom of the opening 502, whereby the diode support 496 can be repositioned depending on the type of media used. When properly assembled with the rest of the components of the printer 60, the media sensor plate 488 is connected to the main control circuit 108 through a suitable opening in the central support wall.

In operation, the illumination source 492 shines through the web of label media 510 so as to respond to the change in the relative opacity of the paper backing 512 and the individual labels 506 at the inter-label gap 514 and the hole 518 or notch 520 separating the tickets or labels 508. In an alternative embodiment (not shown), the illumination source 492 reflects light from one side of the media web 87 and the photodetector 494 is disposed on the same side of the media to respond to a printed visible indicia on the media. When reading the following description, the manner and method of making and using this alternative embodiment will be apparent to any person skilled in the art, it being intended that both embodiments fall within the scope of the appended claims.

The photodetector 494 converts the received light into a variable voltage. The presence of the gap 514, hole 518, or notch 520 produces a signal voltage that is distinct from that of the rest of the medium path 87. Known methods of processing this signal voltage include comparison with a DC voltage and analog-to-digital (A/D) conversion.

Processing by comparison with a DC voltage is simpler, less expensive, and does not require any software processing. The signal voltage is applied to one input of an analog comparator. A fixed threshold voltage having a value between the voltage of gap 514 and label media 518 is applied to the remaining comparator input. The output state of the comparator is an indication of the location of label 506, with the occurrence of a transition being interpreted as passing a label 506 edge. However, the comparison method is susceptible to interference, DC offset errors, temperature effects, and part aging. It also requires manual adjustment in the event of changes in the opacity or reflectivity of the web materials, which vary considerably between manufacturers and production locations. This results in the media sensor 100 potentially being unable to detect the To locate the sticker gap 514 when the illumination level and sensor threshold are not adjustable to accommodate such variations. Previously, this has been accomplished with rheostat adjustment series of the current through the LEDs 492 or with a potentiometer adjustment of the comparator threshold voltage.

The adapted software can perform processing by A/D conversion that is less susceptible to DC offset errors, temperature effects, and part aging. The photodetector voltage is converted by an A/D converter into a numerical value for interpretation by a central processing unit (CPU). The processing is similar to the comparison process discussed above, with the additional step of continuously monitoring the voltage of gap 514 and label medium 510 and calculating the optimal threshold. This adaptive behavior can reduce several errors common in medium sensing, however, limitations in the dynamic range of the available phototransistors 494 may still require manual adjustment of the LED current for some medium materials.

In the invention, the illumination source 492 is automatically adjusted by the medium sensor control board 488 using pulse width modulation so that variations in web opacity and reflectivity are compensated. The voltage response to the transmitted or reflected illumination is independent of ambient light and of changes in the radiant power of the illumination source 492 and in the photodetector 494 operating point due to temperature change or component aging. Accordingly, accuracy comparable to A/D conversion is achieved at a cost closer to a simple comparison. In particular, the illumination source 492 is modulated to provide a reference light intensity and a peak light intensity. A stabilized chopper circuit is employed with the photodetector 494 output for shift error compensation and interference immunity. Referring to FIG-20, a microprocessor 522 includes a timing output capable of generating a clock 524 with a frequency and duty cycle determined by software. During the OFF TIME of the clock 524, a minimum current is flowed through an array of LEDs 492. During the ON TIME of the clock, a charge network formed by a resistor 526 and a capacitor 528 controls the current in the LEDs 492 so that their light input constantly increases during the ON TIME. The LED 492 current and light output return to the minimum level at the ON to OFF transition of the clock 524.

The phototransistor 494 converts all the light received, including any ambient light and light from the LEDs 492 passing through the track, into an electrical signal. A first analog sampling gate 530 (such as an Opto Tran 870 nn) is turned ON to output the electrical signal to a fixed voltage during the OFF TIME of the clock 524. This has the effect of canceling out any shift in the DC offset of the phototransistor circuits and the shift due to ambient light. The output signal is amplified by the first 532 and second 534 operational amplifiers (such as a TLC274) and then again by a second analog sampling gate 536 (such as an Opto Tran 870 nn) to cancel out any DC offset error introduced by the amplifiers. The applied and amplified waveform is then applied to one input of an analog comparator 538 (such as a TLC393). A fixed DC threshold voltage is then applied to the other input of comparator 538. The comparator output state is a logic ONE when the total light received exceeds the reference established during the OFF TIME by an amount proportional to the DC threshold voltage.

A flip-flop circuit 540 latches the output state of comparator 538 at the ON to OFF transition of the timers. The latched state of flip-flop circuit 540 is then fed back to central processing unit 522 as an indication of whether a gap 514, hole 518, or notch 520 is present. The peak light level emitted by LEDs 492 increases as the ON TIME of the timers increases. The photodetector 492 peak voltage excursion from the OFF TIME reference is also greater when the light path passes only the backing 512 alone than when the light path passes the backing 512 and a sticker 506. When the sticker media 510 is changed, a test is performed in which the stickers 506 are passed through the media sensor 100 to evaluate the signal voltage. The ON-TIME of the clock is then selected by the software so that the comparison threshold falls equally between the gap 514 and the label medium 510. If ticket or label medium 516 is used, the medium sensor 100 must be aligned with the notch 520 or hole 518 so that an LED 492 can transmit light directly to the photodetector 494. This is accomplished by adjusting the sensor adjustment arm 502 until direct transmission is established. The calibration procedure then proceeds in the same manner as described with sticker medium 516.

Referring to Fig. 21, a guide peg 430 is shown removed from a cooperating guide ring 432. An engaging end 434 of the guide peg 430 is formed with the locating lugs 436 for engaging a cooperating peg locating hole 438. The engaging end 434 of the guide peg 430 is inserted into the locating hole 438 and rotated (as indicated by arrow 440) to engage the lugs 436 behind a peg flange 442 inside the peg locating hole 438.

The guide post 430 is molded with the engaging end 434 as a single piece of plastic material. On one side of the guide post 430, a convex surface 444 is formed with a smooth surface to facilitate movement of media 87 or ribbon 96 thereon. One end 446 of the guide post engaging end 434 is formed with a partially spherical surface. A reinforcing post 448 is formed on a long side opposite the convex surface to provide support and strength against bending as media 87 or ribbon 96 moves over the convex surface 444.

A number of guide posts 430 are used throughout the printer 30 to guide the media flow and the ribbon to guide and direct the printer during a printing operation. The pegs 430 are quickly inserted and removable for easy setup and easy reconfiguration of the printer for different types of media or ribbon.

In Fig. 22, a media winding or spool spindle 450 is shown. The spindle 450 includes a shaft 452 that extends through a spindle body 454 and the central support wall 74. On the opposite side of the wall 74, as shown in Fig. 22, a spool gear is attached to the shaft 452 and operatively connected to the drive belt driven by the stepper motor 114. In this regard, the spool spindle 450 is driven at the same speed as the platen roller 88 since they are driven by the same source or stepper motor 114. Although the other figures included here do not specifically show the take-up gear or even the shaft 452 from the other side of the wall 74, it can be clearly seen that a projection 458 has been provided on the wall 74 to receive the shaft 452. In addition, it can also be seen that receiving means have been provided by the ribs in the wall 74 so that a suitably sized drive belt can extend along the wall 74 to drive the shaft 52.

In operation, a portion of the media is wound over the spindle so that the center portion overlaps itself to hold the media on the spindle body 454. A wire form spacer 460 extends over the surface of the spindle body 454 to provide a gap between the spindle body surface 462 and the media wound thereon. When the spent media is to be removed from the take-up spindle, a retaining end 464 is released from a retaining hole 466 and pushed axially out from under the wound spent media. Removal of the wire form 460 allows the spent media to be easily removed from the spindle 450.

A spindle full switch 468 is mounted below the spindle 450 to indicate when the spindle needs to be emptied to prevent potential sticking due to excess used media wound around the spindle 450. The spindle full switch 468 includes a sensor arm 470 coupled to a microswitch connected to the control circuit 108. Although the microswitch is not specifically shown here, a microswitch of known construction and mechanical operation that is coupled to a mechanical lever can be used for this purpose. As the used media is wound onto the spindle body 454, the diameter of the roll of used media increases to the point that it strikes the sensor arm 470, thereby displacing the arm and triggering the microswitch, which senses a full condition. A suitable indicator is provided on the printer 60 to indicate to a user that the take-up spindle 450 must be emptied before further operation. In addition, the signal generated by the microswitch triggered by the sensor arm 470 may also be processed by the control circuitry 108 to prevent further operation of the printer 60 until the take-up spindle 450 has been emptied.

SIMPLIFIED PRINT HEAD CONTROL USING A DOUBLE DATA LOAD

Referring now to Figures 50 and 51, in accordance with another inventive feature, a method and apparatus are provided for employing the loading of duplicate data into a thermal printhead to provide improved control of thermal printhead heating. In accordance with this inventive feature of the invention, data is loaded into the serial input of the printhead twice for each row or line of print; i.e., twice for each line of information or annotation to be printed on the media. This results in two heater heating cycles for each line printed. The heaters are selectively charged, with some elements being heated during both cycles and some for only one of the cycles.

According to this inventive feature, the data from the last line printed is used to determine whether a heating element should be heated during the first of these two cycles. Importantly, the existing serial data shift register of the print head holds the data corresponding to the last line of information or the last line of annotation printed, thereby eliminating the need for any external memory to accommodate this inventive feature, so that this feature can be provided at minimal cost.

Generally speaking, the printhead typically used in thermal printing comprises a row of resistive heating elements spanning the width of the intended print media. A single printhead may comprise hundreds of these heating elements with linear densities up to 12 heaters per mm. Digital circuitry, often attached to the printhead substrate, allows the individual resistive heating elements to be selectively activated or heated.

When heated to a predetermined temperature, these elements create an image in the form of a dot on the media, either directly in the case of a heat-sensitive media or through a heat-sensitive ribbon in the case of thermal transfer printing. As the printer feed mechanism or media dispenser moves the media relative to the print head, the bank of heaters is repeatedly loaded with data and activated to create a printed image by repeatedly forming the image from one line of dots at a time. Thus, for a single alphanumeric character, for example, up to 12 lines of information per mm of character height can be printed to form the finished character or other information.

The image or annotation information for a given line comprises binary data, usually in the form of a logic 1, indicating that the heater element is heating, and a logic 0, indicating that the heater element has not yet heated. This data is loaded into a shift register which forms part of the thermal print head. Referring first to Fig. 50, a simplified schematic of a typical thermal printhead is shown and generally designated by the reference numeral 610. The thermal printhead 610 includes a plurality of resistive heating elements 612 which span the width of the intended print medium as described above. The heating elements can be heated via a logic circuit illustrated in FIG. 50, such as a series of corresponding AND gates 614. The AND gates 614 have one input connected to receive a sensor signal at one input end 614 and have a second input connected to receive data from a shift register 618. This shift register forms part of the printhead and is often integrated into the printhead circuitry and/or mounted on the printhead substrate. As illustrated in FIG. 50, an additional inverting buffer 629 is provided between each AND gate 614 and its corresponding heating element 612.

In operation, a given heater 612 is charged if a logic 1 is present at the corresponding data position of the shift register at the same time as a clock pulse signal arrives at input 616. Thus, the data in the shift register actually controls the heating of the heaters 612. The energy applied to these heaters 612 is controlled by the length of the clock pulse signal and by the voltage applied to a common positive voltage input terminal 622. It should be noted that each of the heated heaters receives the same amount of energy since all are connected to the same positive voltage source and all receive the same clock pulse signal when it is released by the data in the shift register. In some cases, however, it is desirable for some of the heating elements 612 to consume more energy than others. For example, if a particular stimulus element has been heated in the previous print line, it will retain some of the energy and therefore require less energy to produce a well-printed dot or image in the immediately following print line. On the other hand, a heating element that has not yet been heated is effectively "cold" and requires somewhat more energy to produce the same dot or image. As print speeds increase, there is less time between print lines and the varying energy requirements of the heating elements, depending on their history, become greater. In addition, overheating of an element can not only degrade the quality of the image but can also result in damage to the heating element. Thus, individual control of the amount of energy applied to each of the stimulus elements 612 is desirable, but is quite difficult due to the design of the thermal printhead as shown in Figure 50 such that all elements receive the same voltage and timing pulse signal.

One prior art control option involves multiple clock pulse cycles per print line. That is, a "hot" element (one that has already been heated) can be activated for only a single clock pulse cycle, while a "cold" heating element (one that has not yet been heated) can be heated for multiple clock pulse cycles. Such an arrangement requires an additional digital memory to store the data of the previous print lines as well as the data for each of the multiple clock pulse cycles. The stored data is used to determine how long it has been since a given heater element was heated, and from this information to determine how many clock pulse cycles the heater element should be heated for to achieve optimal heating. The complexity and cost of such additional digital storage circuitry and decision-making circuitry can be significant, however.

In accordance with an inventive feature, and referring now also to Figure 51, a system is provided in which duplicate data is loaded using only the existing printhead shift register 618. Conveniently, this feature avoids the high cost of additional digital memory and complex decision making circuitry necessary in the prior art approach described above. According to this inventive feature, for each line of imprint to be printed, the data ("print line data") is loaded twice into the printhead shift register. The first charge is referred to as a compensation charge, and the second charge is referred to as a print charge. According to the preferred form of this feature discussed here, in the compensation charge, a digital or logic 1 is loaded into the shift register to heat the elements that have not yet printed on the previously printed line, but are to print on the next print line. Since these heating elements were not heated up on the previous print line, they are considered "cold". a clock pulse is applied, which leads to the charging and warming up of these "cold" heating elements.

The second data or print load then follows immediately. For the print load, the incoming data for the next print line or print line data is loaded into the shift register so that a digital or logic 1 is loaded for each element to be printed on that print line. A clock pulse is then again applied so that each of the elements for which a logic 1 has been loaded is heated, resulting in the desired printed image for that print line. This second load or print load is identical to the data that would be loaded into the shift register if no additional thermal control were used.

The media is then moved to the next print line position and the above process is repeated to produce the desired image or imprint on the media.

The advantage of this inventive feature is that the shift register already present in the printhead is used to store the necessary data. Thus, when the data for the compensation charge is shifted into the printhead, the data for the last line is shifted out. This data is available at the printhead's "Data out" output 624. This output is typically provided for testing the integrity of the shift register 618. According to this inventive feature, the last line of data is shifted out with the new or incoming print line data. combined to produce the desired compensation charge data. The cycle necessary to combine this data to produce the compensation charge is relatively simple and inexpensive.

An embodiment of this feature is illustrated in Fig. 51 as an example. It will be understood that other embodiments may be employed in this regard without departing from the invention. According to the invention, the compensation charge comprises serial data formed according to a rule that states:

producing a data bit for effecting heating of a heating element upon application of a clock pulse signal only if a bit in the print line data corresponding to the last line printed in a given bit position includes a bit that does not initiate heating of a heating element in response to application of a clock pulse signal and a bit of incoming print line data in a bit position corresponding to the given bit position of shift register data is a bit for initiating heating of a heating element in response to a clock pulse signal.

In the embodiment explained, this rule can be expressed somewhat more simply:

Generating a logic 1 bit if a bit of serial data in the shift register in a given bit position is a logic 0 and a bit of incoming data in a bit position corresponding to the given bit position of the shift register is a logic 1, and otherwise generating a logic 0 bit.

As explained in Fig. 51, a switch or switching device 626 is used to select the serial data to be fed into a data input port 628 of the shift register 610. For simplicity of explanation, a mechanical switch has been shown in Fig. 51. However, in practice, a switching device using a digital gate circuit is preferred. This circuit can be implemented using discrete logic, programmable logic, relays and other desired devices.

The above simplified rule is implemented in the illustrated embodiment by using an inverting buffer 630 to receive the data from the data output 624 of the shift register 618 and an AND gate 632 to receive the data from the inverting buffer 630 and also the incoming serial data stream containing the print line data or the information for the next print line. Thus, the AND gate 632 combines the converted data from the last print line as stored in the shift register with the incoming serial data for the next print line to form the appropriate compensation charge according to the above rules. The switch or switching device 626 is then used to select the compensation charge for one cycle and the pressure charge which, according to this inventive feature, is identical to the incoming data for the second cycle of the dual or double data charging cycle. Briefly, the operation is as follows.

Before printing, the print head shift register is initialized by registering logical 0s to fully load the shift register with logical 0s. The printing process is then started and runs in these stages:

1. The switch or switching device 626 is switched to the compensation charge position, i.e. in the preferred embodiment to the output of the AND gate 632.

2. The incoming data is then combined with the AND gate 632, shifting the data out of the shift register 601 and converting it, and the resulting data, which includes the compensating charge, is simultaneously shifted into the shift register 618.

3. The clock pulse signal is activated to thereby heat any heating element for which the appropriate logic is present in the corresponding compensation charge bit in the shift register.

4. The switching device 626 is switched to the pressure charge position in order to directly receive the incoming serial data.

5. The incoming serial data is shifted into the shift register to become the print load.

6. The clock pulse signal is activated, thereby heating the heating elements according to the information or data in the print charge.

7. The media is advanced one line and these steps 1-7 are repeated until the printed image or prints are complete.

The above method and device offer a number of advantages over existing methods and devices, which are generally as follows:

Better print quality is possible at higher speeds than with single-load methods. Costs are lower than with existing multiple-load methods. No external memory components are required. No high-speed data calculations are required. The compensation and print load cycles can be independently adjusted by adjusting the clock pulse timing. Only relatively simple and inexpensive digital logic circuitry is required to implement this feature, with the memory requirements being met by the existing print head shift register.

IMPROVED PRINT QUALITY IN AREAS OF ACCELERATION AND SLOWING

The amount of energy required to print a line or row of an image on a medium varies with the speed of the medium relative to the print head and also with the print head temperature in the case of a thermal print head. Software control boards have historically used several equations to determine the correct length of the pulse width of the clock pulse signal for acceptable printing for a given medium speed and print head temperature. These equations generally take the form of a series of simultaneous equations of the following form:

Pulse width = BPWn · Kn (instantaneous printhead temperature)

where BPWn is the base pulse width (in units of time) for a given instantaneous media velocity relative to the printhead and Kn is a gain constant that determines how much the base pulse width must be increased or decreased based on the instantaneous printhead temperature. Most applications use an equation for a constant media velocity relative to the printhead. This method produces acceptable results while the velocity remains constant. However, print quality in areas of media acceleration or deceleration may not be acceptable because the equations calculate pulse widths based on the desired constant velocities rather than the instantaneous velocity during acceleration or deceleration.

Attempts have been made to solve this problem by reducing the size of the acceleration and deceleration regions on the media, however due to mechanical limitations this also reduces the amount of printable area on the media. In addition, the smaller these regions of acceleration and deceleration are, the more media slippage and tracking problems occur. These problems become worse with decreasing media sizes, i.e. where relatively small stickers, tickets, labels etc. are to be printed.

According to the invention, an individual base pulse width (BPW) and a value for the head temperature gain constant (K) for each instantaneous velocity of the media relative to the print head. This results in the establishment of a separate pulse width equation of the above general formula for each possible instantaneous velocity. Since the pulse width can now be tuned to any instantaneous velocity, the print quality in the regions of acceleration and deceleration can be made to approximate or equal to that of the regions of constant velocity. Accordingly, the size of these regions of acceleration and deceleration can be increased without loss of print quality, thereby solving many of the mechanical problems caused by reducing the size of these regions and related problems, particularly those of the relatively small size of the tickets, labels, stickers or other media as noted above.

Two serious limitations have prevented the use of this type of solution in the past. A first limitation involves the use of floating point arithmetic to obtain the solution required for each equation. If the pulse width for each step must be calculated while the printer is printing, there is not enough time for a low-cost processor to perform the required floating point calculations. The second problem concerns the length of development time required to "fine-tune" the values to be used in each equation. In the past, it has been found that a skilled person needs about a day to fine-tune a single equation for constant printing speeds as given above. However, the proposed method can require five to ten times the number of equations used in the case of constant printing speeds.

According to the invention, a table is prepared containing the values for the base pulse width (BPW) and head temperature gain constant (K), each value corresponding to a constant speed supported by the printer. These values generally correspond to those used in the equation described above. The BPW values are given in units of time and the Kn values are given in units of percent BPW per unit of temperature.

When power is initially applied to the printer and before the printing process begins, the two tables of BPW and K values are set up using floating point arithmetic. This then avoids the problem of attempting to calculate the values during the printing process. The number of values in each table is one more than the number of increments of speed that the printer's medium dispensing mechanism can support up to and including its maximum speed. Floating point arithmetic is then used to interpolate the values in each table, carefully scaling the values where necessary to avoid loss of accuracy.

When starting the printing process, a test print run can be used to fine-tune the print quality. During this test run, the print quality is recorded. The values in the BPW and K tables above are varied during printing at least at a constant rate until the recorded print quality is acceptable. A floating point arithmetic operation then calculates the values for the rest of the table entries.

Then, during actual printing, the pulse width of the clock pulse signal is calculated using the following equation:

Pulse width = BPW_TABLE [1]*K_TABLE [1]*HEAD_TEMPERATURE,

where i represents a given increment of the current speed toward a constant speed supported by the printer.

FEATURE OF THE SEGMENT COMMAND

The process for printing a label is explained by the block diagram of Fig. 52. The process includes three sub-processes, P1, P2 and P3. A typical label and some typical features are shown in Fig. 53.

A state-of-the-art multitasking technique used by the CPU allows the three subordinate processes of Fig. 52 to be executed simultaneously. Each process is executed sequentially for a maximum time interval called a time slice. When the time slice has expired, the process is stopped and t is stored for later resumption in the same state it was in when the time slice expired.

When a process is running, the execution flow is shown by the solid lines of Fig. 52 in the usual manner. The processes run on data stored by one of the other processes in a RAM memory according to the state of the art common to both.

The process for printing a label begins with receiving characters from a host computer. These are processed when process P1 next executes its stage S1. The characters include interspersed commands and data written in a label description language recognized by the printer.

The characters are stored in a state-of-the-art buffer in stage S2. A loop between stage S3 and stage S1 repeats until stage S3 determines that the contents of the buffer include a field that completely describes a text, bar code, graphic or other object to be printed. The contents of the field include, without limitation, location, size, data content and other information required to define the object. Whenever a complete field is detected in stage 3, it is passed on to process P2 as data input.

When process P2 is executed next, stage S5 determines whether a field has been entered by process P1. If so, the point image of the specified object written into a state-of-the-art bit allocation memory at the desired location.

Referring to Figure 53, the commands in the label description 1 may include one or more occurrences of a segment command that divides the corresponding label 2 into one or more subdivisions 3. The first such segment command 4 defines a first subdivision 5 of the label 2, which the printer is free to print upon receiving the first segment command 4. The first segment command 4 signals that the previous commands and data sent to the printer completely define the objects in the first subdivision 5, that no further commands relating to the objects in the subdivision are to be expected, and that the printer may begin printing subdivision 5 or advance that subdivision when it is reached.

The second segment command 5 defines a second subdivision 7 of the sticker 2, the printing of which is up to the printer if it subsequently receives the second segment command 7. The sticker description 1 can contain a large number of segment commands within the scope of the claims.

Referring to Figure 52, method P2 describes point images of fields into a bit map memory for as many fields as are available from process P1 or until a segment command is reached. When a segment command occurs, the complete partition is sent as input data to process P3.

When process P3 is next executed, stage S9 determines when a complete division has been reached. If so, the printing process begins at step 510 and continues until the end of the division or until the end of the label, whichever is encountered first.

Referring to Figure 36, the printer is controlled by a single MC68331 microprocessor. It is a 32-bit surface mount processor, with a 32020 processor core, interrupt controllers, counters/timers, and programmable chip select lines. The DRAM basic control functions are also included. The processor uses a 32.768 kHz clock crystal as a reference. An internal synthesizer multiplies the reference to obtain the 16 MHz operating clock.

The reset circuit (207) provides an active LCW state for 15 ms after the voltage rise. This allows for stabilization of the clock and initialization of the internal registers. The RESET* line is provided as an open collector type, which is also driven by the processor to enable a software initiated reset.

The system firmware contains service test programs that are useful for troubleshooting and setting up the printer. Test mode is activated by powering on the printer, with TP1 and TP2 linked together (2C8) The connector W1 is only used during PCB manufacturing to enable burn-in tests. W1 should not be installed in the field.

As shown in Fig. 37, the standard printer contains four 256K x 4 DRAM los for a total of 512 KB. The los are soldered on locations U1, U3, U5 and U7. An additional 512 KB can be mounted on sockets U2, U4, U6 and U8. The DRAM control lines are programmable output lines on the processor (2C1), (2D1), (2D8). GAL U9 decodes the DRAM control lines to generate the RASx* and CASx* signals as well as the ROW*/COL line for the multiplexers U11 and U12.

Referring to Figure 38, the system firmware is located in EPRONS or mask ROMs in sockets at locations U13 through U16. The chip selects are provided by programmable chip select outputs on the processor (2D1). The system configuration is stored in EEPROM U26 (4B7). The EEPROM provides a direct interface to the I/O lines from the processor.

A head open circuit is shown in Fig. 39. As shown in Fig. 39, the main board contains a phototransistor (Q1) pointing to an IR LED (D1) (5B5). The head mechanism has an opaque mask that breaks the light path when the head is pushed in. The collector voltage of Q1 is sensed by comparator U22B. The comparator reference is set to 2.5V by R59 and R60. The comparator output HDOPEN* is connected to an interrupt input of the processor (2C8). When the head is not pushed in, light from D1 saturates Q1. The Q1 collector drops to a few tenths of a volt and drives HDOPEN* LOW. R67 provides positive feedback to eliminate switching noise.

The sticker taker sensor, as shown in Fig. 39, consists of a phototransistor located opposite an IR LED. They are mounted just outside the tear-off edge so that a dispensed sticker refracts the light beam. The sensor is connected to JE (5B1). The NPN phototransistor is connected to the collector at Vcc and to the emitter to R64. The signal is applied to the comparator V220 (5B3). The comparator reference is set to 2.5V by REP and REO. The comparator output LBLTKN is applied to an input of the processor (2B8). When a sticker is dispensed, the light beam is refracted and turns the phototransistor OFF. The emitter voltage is less than 1V. When the sticker is removed, the phototransistor turns ON and forces LBLTKN HIGH. R66 provides positive feedback to eliminate switching noise.

The serial port configuration and other modes of operation are set by an 8-position DIR switch connected to the BD-25 connector. The processor reads the switch setting as a serial bit stream from the parallel in/serial out shift register V20. The serial switch data (DIPDAT) and the shift clock (DIPCLK) are driven by the processor (2D8). The DIP shift switch shares the I/O pins with the LED indicator. Since the DIP switch is only read when the power is on, no conflicts occur.

The front panel contains 8 LEDs and 4 pushbutton switches. It is connected to the logic board via 10-way ribbon cable. The pushbuttons (5D5) are connected to the individually sampled inputs on the processor (2C8). The LEDs are driven by a serial in/parallel out shift register U34. The serial LED data (LEDDAT) and the shift clock (LEDCLK) are driven by the processor (2D8).

Referring to Fig. 40, the print head drive circuit consists of a FIFO (U17) to parallelize the data, a GAL (U24) and flip-flop (U25) for control, and a buffer (U23) to drive the head lines. The head cable mates with J3.

The print head charge and the clock pulse cycle are synchronized to each half-step of the motor. Two half-steps are performed for each print line. Therefore, the print head is charged and clocked twice per print line.

At the start of a load cycle, U17 (6B6) is loaded with 52 words of print data (832 bits) via its parallel input. HDCTL (6D8) is set to LCW for the first load cycle. FCLKEN* is set to LOW, followed one clock cycle later by HCLKEN*. The print head data (NEWDAT) is shifted out of U17 and combined with the previous data from the head (OLDDAT). The data streams are combined in U23 (6B5) and sent to the print head (HEADDAT) via U23 (6D4) together with the shift clock (HDCLK). The contact line (HLATCH*) is pulsed LOW, followed by the print pulse (HSTRB*). The length of HSTRB* determines the blackness of the print. The entire process is repeated during the second half-stage, except HDCTL is kept HIGH, which causes HEADDAT to be processed differently.

The timing is controlled by counters in the processor. The counters are driven by a 4 MHz clock CLK4 (6D8). The 16 MHz clock (CLK16) is halved by U25B (6C7) to make CLKB. U24 further divides CLKB to make CLK4.

The processor compensates for head and feed losses when many dots are activated in a line. The head data is applied to a 1-bit counter in U64. Each count at the output, CNTX2, represents two dots turned ON. CNTX2 is halved again in U25A to produce PECNT, which is applied to a counter in the processor (2B8). The processor adjusts the HSTRB* pulse according to the count accumulated during head loading.

The printhead heat sink temperature is recorded by a thermistor. The thermistor has a negative temperature coefficient with a resistance of 30 KOhm at 25ºC. The heat sink temperature is determined by measuring the time required to charge a capacitor through the thermistor resistance. The TEMPCTL (6A8) is normally HIGH, which turns the open collector output of U21A ON (6A4). U21A keeps C40 in the discharged state (OV). The processor begins a measurement by setting TEMPCTL LOW and activating an internal timer. U21A is turned OFF, and C40 is charged by the thermistor. The comparator U21B stops the processor timer when the voltage at C40 reaches 2.5 V. The processor reads the elapsed time and calculates the temperature. Higher temperatures result in shorter charging times.

Referring to Fig. 41, the serial interface input is built into the processor. It provides a standard UART interface with hardware acknowledgement at TTL signal levels. U27 converts the TTL signals to RS232 standards. The chip contains charge pumps to generate ±10 from the Vcc voltage supply. R43 forces RTS to always be active. Hardware acknowledgement is controlled with DTR and DSR.

The sensors use a chopper-stabilized design, which provides stability, a wide operating range and resistance to ambient light. The sensitivity of the sensor is adjusted by adjusting the LED light source. The adjustment is done by software control of a PWM (pulse width modulation) signal from the processor. The PWM repetition controls the speed of the chopping process, while the duty cycle controls the sensitivity (see Fig. 54)

The media and tape sensors are located on a separate PC assembly and are described below.

The sensor amplifiers and detectors are located on the logic board and are described below. Since the MEDIUM and BAND circuits are similar, only the MEDIUM circuit is discussed.

Referring to Figures 42 and 44, the high gain sensor amplifiers use an isolated ground plane on the logic board and a separate (+5F) supply to suppress noise. The sensor ground is connected to the logic ground through W3 (11A7). The +5F supply is controlled by U31 (11B4). The sensor assembly is connected to the logic board at JE (11C6).

The sensor output is a 7.8 kHz sawtooth waveform with approximately 15 mV peak amplitude when a track is detected. The sensor amplifier consists of two operational amplifiers, U30A and U30B, each having a voltage gain of 19 for a total gain of 361 (51 dB). The band sensor gain factor is 121 (42 dB). The amplified signal is applied to the comparator U33A. The comparator output is sampled by U32A at the end of each PWM cycle to provide a stable signal to the processor. The comparator input voltage (U33A, pin 3) is +5 V, with no light through the medium. The comparator threshold is set to 4.1 V by R91 and R92. Increasing light causes the comparator input to decrease. When the light intensity drives the comparator input below 4.1 V, the comparator output goes LOW. The output is stored in a flip-flop, causing MEDIUM* to go HIGH at the end of the cycle. The MEDIUM* line is read by a sampled input on the processor (2C8).

The amplifiers are stabilized by self-zeroing during the time PWM is LOW. The sampling gates U29A and U29B are turned ON and connect U30A pin 3 and U33A pin 3 to the +5F supply. The input capacitor C55 charges depending on the ambient light level (LEDs at minimum output). The output capacitor C56 discharges to 0 on itself and holds the compare input at the +5F supply. When MPWM goes to HIGK, the sampling gate turns OFF and allows the amplifiers to operate. The LED output rises until MPWM goes back to LOW. The rising waveform from the sensor output is amplified.

A tape spin motor circuit is shown in Fig. 44. The tape take-up spindle is driven by a DC motor whose torque is electronically adjusted. The motor is driven by an adjustable DC voltage regulator. Section 1 of the dual timer U19 operates as a 6 kHz oscillator. Section 2 is a single pulse triggered by the oscillator. The output of section 2 is a continuous pulse train whose duty cycle is adjustable from 15% to 25%. Section 2 drives the power FET Q2 and provides power to the motor. The free wheel current continues to flow through D3 when Q2 turns off. The regulation occurs because the timing components of section 2 are driven by VHEAD, not Vcc. As VHEAD is increased, the duty cycle of the FET is correspondingly decreased.

The sensor assembly is described with reference to Figs. 41 and 54. The LEDs are driven by a ramp generator consisting of Q1 and Q2. Q3 holds the ramp generator at OFF and the LED current at a minimum while MWPM is LOW. The sensor senses a low level reference light while the amplifiers automatically adjust to 0. When MPWM goes HIGH, the LED current and brightness increase linearly. The processor adjusts the LED brightness by controlling the duty cycle (ON time) of the MPWM.

The phototransistor PT1 detects the LED light passing through the medium. PT2 is not used. Q7 and the diodes D1 and D2 set the operating point for PT1. The potentiometer RV1 allows the gain adjustment. The sensor output is buffered by Q8. The output waveform is a small sawtooth voltage (10 mV) on a large DC bias (up to 2 V depending on the setting of RV1). The sawtooth voltage component is amplified and used. The DC component, including the ambient light, is rejected by the sensor amplifiers.

Fig. 55 is an exploded perspective view of the power supply circuit 128 removed from the base recess 140. The power supply circuit 128 also includes a circuit board opening 586 with a switch circuit or wire bond 588 soldered across it. Wire bond 588, also shown as switch wire JMP1 on the power supply circuit of Fig. 46, is soldered to the power supply circuit 128 at at least first and second points or pads of the circuit board 590 and forms part of the voltage selection circuit of the power supply circuit 138.

Fig. 56 also shows a splitting device 592 and a short plug 594 made of plastic or an equivalent electrically insulating material, one or the other of which is inserted into an opening 586 in deck 154. The splitting device 592 includes a head end 598 and a splitting end 600. The splitting end 600 has a retaining portion 602, such as spikes, extending outwardly and engaging an interior surface of the base foundation 140 and surrounding the control opening 596. The splitting device 592 and the socket device 594 are shaped and constructed for a snap-in interference fit with a control opening 596 to make it difficult to remove the splitting device 592 once it has been inserted and snapped into place.

The plug 594 does not extend under the deck 154 when inserted into the opening 586 and does not come into contact with the power supply circuit 138. It serves to prevent probes or tools inserted into the opening 586 from coming into contact with a wire cable 588 or other electrical components.

The splitter 592 is dimensioned to extend through the opening 586 into a power supply circuit 138, thereby interrupting the cable 588 to permanently change the voltage setting of the circuit 138. The splitter 592 also remains in a gap created when the cable 588 is separated to isolate the interrupted ends of the cable 588.

Fig. 56A is a detailed view of the power supply circuit 138 after insertion of the dividing device 592.

Claims (2)

1. On-demand printer (60) of the type used for printing tickets (508), labels (508), adhesive labels (2, 506) and other media (87), the printer (60) having various components and comprising: a structure (73) for supporting and enclosing the components; a power supply circuit (138) for receiving power from an external source and adjusting the power to operate the printer (60); an input device (618) for receiving command signals related to the operation of the printer (60); a control circuit means (108) mounted on the structure (73) and coupled to the input means (618) and the power supply circuit (138) for processing the command signals and generating the corresponding control signals for operating the printer (60); a print head device (84) mounted on the structure (73) and coupled to the control circuit device (108) for receiving the control signals from the control circuit device (108) and for printing characters on the medium (87); a media output device (86) operably connected to the printhead device (84) and coupled to the control circuit device (108) for moving the media (87) to the printhead device (84) in response to the control signals; a slip clutch assembly (396) operatively attached to the media dispenser (86) for use with a rotating shaft (398) to maintain a constant tension on a belt (96), a spiral spring (402) of the slip clutch assembly (396) having a foot portion (414) extending from one end of the spiral spring (402); a collar (404) of the slip clutch assembly (396) having a bore (408) therein for receiving the spiral spring (402), the foot portion (414) extending from the collar (404), the rotary shaft (398) extending through the spiral spring (402) in the collar (404) and the collar (404) being connectable to the rotary shaft (398); and a plurality of inclined teeth (142) positioned around the rotary shaft (398), the plurality of inclined teeth (142) each having an inclined surface, the base portion (414) of the coil spring (402) controllably engaging the inclined teeth (142). 2. A slip clutch assembly (396) for use with a rotating shaft (398) for maintaining a constant tension on a belt (96), the slip clutch assembly (396) comprising: a coil spring (402) having a foot portion (414) extending from one end of the coil spring (402); a collar (404) having a bore (408) therein for receiving the spiral spring (402), the foot portion (414) extending from the collar (404), the rotary shaft (398) extending through the spiral spring (402) in the collar (404) and the collar (404) is attachable to the rotary shaft (398); and a plurality of inclined teeth (142) positioned around the rotary shaft (398), the plurality of inclined teeth (142) each having an inclined surface, the base portion (414) of the coil spring (402) controllably engaging the inclined teeth (142).